fix: harden flatfile backend, make SQLite optional

Build system:
- Add --without-sqlite configure flag (AM_CONDITIONAL BUILD_SQLITE)
- Guard database_sqlite.c with HAVE_SQLITE in Makefile.am, database.c,
  database.h and stub_database.c
- Fall back to flatfile when SQLite not compiled in

Security (database_flatfile.c):
- MAM dedup: skip incoming messages with duplicate stanza-id (archive_id)
- LMC sender validation: reject corrections from mismatched JIDs
- Add flock() advisory locking to prevent interleaved writes from
  concurrent instances
- Check fprintf return when writing file header

Autocomplete (cmd_ac.c):
- Add 'flatfile' to /privacy logging autocomplete
- Add dedicated /history autocomplete with on/off/verify (was boolean-only)

Code quality:
- Fix fwrite return type: size_t not ssize_t (database_flatfile_parser.c)
- Fix mixed allocators in ff_readline: use malloc() consistently for
  overlength-line fallback instead of g_strdup()
- Fix index alignment in _ff_state_extend: use local counter so index
  step doesn't depend on total_lines from initial build

Documentation:
- Fix page: correct directory structure (history.log not per-day files)
- Fix page: add aid:{archive_id} to line format example
- Fix page: missing newline before .SH BUGS

src/database_flatfile_parser.c

@@ -369,8 +369,12 @@ ff_readline(FILE* fp, gboolean* truncated)
         }
         if (truncated)
             *truncated = FALSE;
-        // Return empty string so caller's loop continues (parse will reject it)
-        return g_strdup("");
+        // Return empty string so caller's loop continues (parse will reject it).
+        // Use malloc() so callers can always use free() consistently.
+        char* empty = malloc(1);
+        if (empty)
+            empty[0] = '\0';
+        return empty;
     }
     if (truncated)
         *truncated = FALSE;
@@ -450,9 +454,9 @@ ff_write_line(FILE* fp, const char* timestamp, const char* type, const char* enc
                     timestamp, meta->str, sender->str, safe_msg);
 
     size_t to_write = full_line->len;
-    ssize_t written = fwrite(full_line->str, 1, to_write, fp);
-    if (written != (ssize_t)to_write) {
-        log_error("flatfile: partial write (%zd/%zu)", written, to_write);
+    size_t written = fwrite(full_line->str, 1, to_write, fp);
+    if (written != to_write) {
+        log_error("flatfile: partial write (%zu/%zu)", written, to_write);
     }
 
     g_string_free(full_line, TRUE);