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

Makefile.am

@@ -3,7 +3,6 @@ core_sources = \
 	src/log.c src/common.c \
 	src/chatlog.c src/chatlog.h \
 	src/database.h src/database.c \
-	src/database_sqlite.c \
 	src/database_flatfile.c src/database_flatfile.h \
 	src/database_flatfile_parser.c \
 	src/database_flatfile_verify.c \
@@ -259,6 +258,12 @@ core_sources += $(omemo_sources)
 unittest_sources += $(omemo_unittest_sources)
 endif
 
+sqlite_sources = src/database_sqlite.c
+
+if BUILD_SQLITE
+core_sources += $(sqlite_sources)
+endif
+
 all_c_sources = $(core_sources) $(unittest_sources) \
 				$(pgp_sources) $(pgp_unittest_sources) \
 				$(otr4_sources) $(otr_unittest_sources) \