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.c

@@ -444,7 +444,7 @@ _flatfile_init(ProfAccount* account)
         g_hash_table_destroy(g_contact_states);
     }
     g_contact_states = g_hash_table_new_full(g_str_hash, g_str_equal,
-                                              g_free, (GDestroyNotify)ff_state_free);
+                                             g_free, (GDestroyNotify)ff_state_free);
 
     log_info("Initialized flat-file database backend: %s", base_dir);
     return TRUE;

src/database_flatfile_parser.c

@@ -372,7 +372,8 @@ ff_readline(FILE* fp, gboolean* truncated)
         // 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';
+        if (empty)
+            empty[0] = '\0';
         return empty;
     }
     if (truncated)