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

configure.ac

@@ -94,8 +94,17 @@ PKG_CHECK_MODULES([curl], [libcurl >= 7.62.0], [],
     [AC_CHECK_LIB([curl], [main], [],
         [AC_MSG_ERROR([libcurl 7.62.0 or higher is required])])])
 
-PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.22.0], [],
-    [AC_MSG_ERROR([sqlite3 3.22.0 or higher is required])])
+### sqlite (optional — can be disabled with --without-sqlite)
+AC_ARG_WITH([sqlite],
+    [AS_HELP_STRING([--without-sqlite], [build without SQLite support (flat-file backend only)])],
+    [], [with_sqlite=yes])
+
+AS_IF([test "x$with_sqlite" != "xno"],
+    [PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.22.0],
+        [AC_DEFINE([HAVE_SQLITE], [1], [SQLite support])],
+        [AC_MSG_ERROR([sqlite3 3.22.0 or higher is required (use --without-sqlite to disable)])])],
+    [AC_MSG_NOTICE([Building without SQLite — flat-file backend only])])
+AM_CONDITIONAL([BUILD_SQLITE], [test "x$with_sqlite" != "xno"])
 
 ACX_PTHREAD([], [AC_MSG_ERROR([pthread is required])])
 AS_IF([test "x$PTHREAD_CC" != x], [ CC="$PTHREAD_CC" ])
@@ -416,6 +425,7 @@ AC_OUTPUT
 AC_MSG_NOTICE([Summary of build options:
 PLATFORM                   : $target_os
 PACKAGE_STATUS             : $PACKAGE_STATUS
+SQLite support             : $with_sqlite
 GTK_VERSION                : $GTK_VERSION
 LIBS                       : $LIBS
 Install themes             : $THEMES_INSTALL