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.c

@@ -41,6 +41,7 @@
 
 #include "log.h"
 #include "common.h"
+#include "config.h"
 #include "database.h"
 #include "config/preferences.h"
 #include "ui/ui.h"
@@ -69,8 +70,13 @@ log_database_init(ProfAccount* account)
         active_db_backend = db_backend_flatfile();
         log_info("Using flat-file database backend");
     } else {
+#ifdef HAVE_SQLITE
         active_db_backend = db_backend_sqlite();
         log_info("Using SQLite database backend");
+#else
+        log_info("SQLite not compiled in, falling back to flat-file backend");
+        active_db_backend = db_backend_flatfile();
+#endif
     }
 
     if (!active_db_backend) {