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

docs/profanity.1

@@ -215,17 +215,18 @@ Or use the command:
 Flat-file logs are stored under
 .IR $XDG_DATA_HOME/profanity/flatlog/ ,
 organized as
-.IR {account_jid}/{contact_jid}/{YYYY_MM_DD}.log .
+.IR {account_jid}/{contact_jid}/history.log .
 .PP
 Each line has the format:
 .br
 .EX
-{ISO8601} [{type}|{enc}|id:{id}|corrects:{id}] {sender}: {message}
+{ISO8601} [{type}|{enc}|id:{id}|aid:{archive_id}|corrects:{id}] {sender}: {message}
 .EE
 .PP
 Use
 .B /history verify
-to check integrity of stored history (both SQLite and flat-file)..SH BUGS
+to check integrity of stored history (both SQLite and flat-file).
+.SH BUGS
 Bugs can either be reported by raising an issue at the Github issue tracker:
 .br
 .PP