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

tests/unittests/database/stub_database.c

@@ -23,6 +23,7 @@
 #include <glib.h>
 #include "prof_cmocka.h"
 
+#include "config.h"
 #include "database.h"
 
 db_backend_t* active_db_backend = NULL;
@@ -66,11 +67,13 @@ integrity_issue_free(integrity_issue_t* issue)
         g_free(issue);
     }
 }
+#ifdef HAVE_SQLITE
 db_backend_t*
 db_backend_sqlite(void)
 {
     return NULL;
 }
+#endif
 db_backend_t*
 db_backend_flatfile(void)
 {