feat(history): flat-file backend with bidirectional SQLite migration

A flat-file alternative to the SQLite chatlog backend with runtime
switching, full migration tooling, integrity verification, and a
synthetic load harness. SQLite remains the default; both backends share
one dispatch layer (db_backend_t vtable) so callers don't change.

Storage layout
- Per-contact append-only `flatlog/<account>/<contact>/history.log`
  under XDG_DATA_HOME, one line per message
- Single-line file header with embedded format-version marker
  (FLATFILE_FORMAT_VERSION); reader warns on missing or mismatched
  marker, writer and checker stay in sync via preprocessor
  stringification
- Deterministic key=value metadata (`id`, `aid`, `corrects`, `to`,
  `to_res`, `read`) plus escaped body \u2014 `\|`, `\]`, `\\`, `\n`, `\r`
  literals prevent log injection
- Sparse byte-offset index (FF_INDEX_STEP=500) per contact for
  O(log n) time-range lookups; rebuilt on inode / size / mtime
  change, extended in-place when the file just grew
- Per-contact GHashTable caches for archive_id presence and
  stanza_id \u2192 from_jid mapping (O(1) MAM dedup, O(1) LMC sender
  validation)

Hardening
- Path-traversal protection: JID directory name normalisation
  (`@` \u2192 `_at_`, slashes and `..` rejected at construction); every
  per-contact path is anchored under the account's flatlog/
  directory and validated before open
- Symlink-attack protection: every fopen / open uses O_NOFOLLOW; on
  ELOOP the operation aborts with an error rather than following
- Filesystem permissions: log files created with mode 0600,
  directories with mode 0700; both enforced at creation, verified
  on each open and reported on drift by `/history verify`
- Atomic crash-safe export: write to a temp file via mkstemp (mode
  0600, random suffix, no name collisions between concurrent
  exports), fsync, then rename \u2014 partial state never replaces the
  live file
- Concurrency: advisory flock(LOCK_EX) held for the duration of
  every write, including append from live messages and full rewrite
  from export, so two profanity processes can't interleave bytes
  on the same log
- DoS / abuse guards:
    * FF_MAX_LINE_LEN = 10 MB \u2014 lines longer than this are rejected
      at read with a warning; the parser will not allocate
      unbounded memory for a single record
    * FF_MAX_LMC_DEPTH = 100 \u2014 `corrects:` chain walk stops at this
      depth and emits a warning, preventing a malicious correction
      cycle from spinning the apply pass
    * FF_VERSION_SCAN_MAX = 16 \u2014 header version probe never reads
      past 16 leading comment lines, even on garbage input
    * Empty / inverted byte-range early-return in page-up read path
      so a malformed time filter cannot cause an unbounded scan
    * Zero-entry index guard so a file whose every line failed to
      parse cannot cause a NULL deref on later page-up
- LMC sender validation: an incoming correction whose sender does
  not match the original message's sender is rejected at write
  time and surfaced via cons_show_error; a cycle in the apply pass
  is broken via a visited-set
- jid_create_from_bare_and_resource treats NULL, empty string, and
  the literal "(null)" as no resource and returns a bare jid;
  similar normalisation for barejid eliminates the legacy
  "user@host/(null)" artefact that leaked into stored fulljids
  whenever g_strdup_printf("%s", NULL) ran inside create_fulljid

Commands
- `/history switch sqlite|flatfile` \u2014 runtime backend swap, closes
  the old backend and opens the new one without reconnecting
- `/history export [<jid>]` \u2014 SQLite -> flat-file, merging with any
  existing flatlog (dedup keyed on a SHA-256 hash mixing stanza_id,
  timestamp, from_jid, body \u2014 robust against id reuse by older
  clients)
- `/history import [<jid>]` \u2014 flat-file -> SQLite, same merge
  semantics, runs inside a single SQLite transaction with rollback
  on per-contact failure
- `/history verify [<jid>]` \u2014 integrity check; emits a structured
  list of issues (ERROR / WARNING / INFO) per file:
    * file-level: missing log, wrong permissions (\u2260 0600), UTF-8
      BOM present, CRLF line endings, empty file
    * line-level: invalid UTF-8 (with byte offset), embedded
      control characters, unparsable lines, timestamps out of
      order, duplicate `id:` and `aid:` (tracked separately so a
      stanza/archive id collision isn't double-reported)
    * cross-line: broken `corrects:` references whose target id is
      not present in the file
- `/history backend` \u2014 show currently active backend
- Active backend indicator `[sqlite]` / `[flatfile]` in the status
  bar next to the JID
- Roster-JID autocomplete for verify / export / import
- export and import open a SQLite handle on demand when the
  flatfile backend is currently active, so migration works
  regardless of which backend is live

Tests
- Unit: database_export (parser round-trip, escape/unescape, dedup
  key stability, JID normalisation), database_stress (14 cases
  exercising rapid writes, large messages, deep LMC chains, MAM
  dedup, concurrent contacts)
- Functional: history persistence across reconnects, export /
  import round-trip with content equality, MUC migration,
  timestamp normalisation across timezones
- Bench harness P1\u2013P5 (synthetic load: bulk insert, time-range
  read, page-up scroll, MAM ingest, mixed workload) and failure
  modes F1\u2013F17 (page-up cursor and forward-iteration symmetry,
  oversized lines, MAM dedup, LMC depth and cycles, BOM/CRLF,
  missing log, empty file, mtime+inode flip, broken corrects, etc.)
- All bench tests integrate with the existing make targets and
  emit CSV rows for baseline comparison

Author: jabber.developer2 <jabber.developer2@jabber.space>
Reviewed-by: jabber.developer <jabber.developer@jabber.space>

src/command/cmd_ac.c

@@ -275,6 +275,8 @@ static Autocomplete logging_ac;
 static Autocomplete logging_group_ac;
 static Autocomplete privacy_ac;
 static Autocomplete privacy_log_ac;
+static Autocomplete history_ac;
+static Autocomplete history_switch_ac;
 static Autocomplete color_ac;
 static Autocomplete correction_ac;
 static Autocomplete avatar_ac;
@@ -429,6 +431,8 @@ static Autocomplete* all_acs[] = {
     &logging_group_ac,
     &privacy_ac,
     &privacy_log_ac,
+    &history_ac,
+    &history_switch_ac,
     &color_ac,
     &correction_ac,
     &avatar_ac,
@@ -1138,6 +1142,18 @@ cmd_ac_init(void)
     autocomplete_add(privacy_log_ac, "on");
     autocomplete_add(privacy_log_ac, "off");
     autocomplete_add(privacy_log_ac, "redact");
+    autocomplete_add(privacy_log_ac, "flatfile");
+
+    autocomplete_add(history_ac, "on");
+    autocomplete_add(history_ac, "off");
+    autocomplete_add(history_ac, "backend");
+    autocomplete_add(history_ac, "switch");
+    autocomplete_add(history_ac, "verify");
+    autocomplete_add(history_ac, "export");
+    autocomplete_add(history_ac, "import");
+
+    autocomplete_add(history_switch_ac, "sqlite");
+    autocomplete_add(history_switch_ac, "flatfile");
 
     autocomplete_add(logging_group_ac, "on");
     autocomplete_add(logging_group_ac, "off");
@@ -1776,7 +1792,7 @@ _cmd_ac_complete_params(ProfWin* window, const char* const input, gboolean previ
 
     // autocomplete boolean settings
     gchar* boolean_choices[] = { "/beep", "/states", "/outtype", "/flash", "/splash",
-                                 "/history", "/vercheck", "/privileges", "/wrap",
+                                 "/vercheck", "/privileges", "/wrap",
                                  "/carbons", "/slashguard", "/mam", "/silence" };
 
     for (int i = 0; i < ARRAY_SIZE(boolean_choices); i++) {
@@ -1786,6 +1802,24 @@ _cmd_ac_complete_params(ProfWin* window, const char* const input, gboolean previ
         }
     }
 
+    result = autocomplete_param_with_ac(input, "/history switch", history_switch_ac, TRUE, previous);
+    if (result) {
+        return result;
+    }
+
+    gchar* history_jid_subcmds[] = { "/history verify", "/history export", "/history import" };
+    for (int i = 0; i < ARRAY_SIZE(history_jid_subcmds); i++) {
+        result = autocomplete_param_with_func(input, history_jid_subcmds[i], roster_barejid_autocomplete, previous, NULL);
+        if (result) {
+            return result;
+        }
+    }
+
+    result = autocomplete_param_with_ac(input, "/history", history_ac, TRUE, previous);
+    if (result) {
+        return result;
+    }
+
     // autocomplete nickname in chat rooms
     if (window->type == WIN_MUC) {
         ProfMucWin* mucwin = (ProfMucWin*)window;