feat: add flat-file database backend for message history

Add pluggable storage backend abstraction (vtable) to the database layer,
allowing selection between SQLite (default) and a new flat-file backend
that stores messages as human-readable plain text files.

New files:
- database_sqlite.c: extracted SQLite backend from database.c
- database_flatfile.c: plain text backend with tolerant parser,
  LMC correction chains, UTF-8/BOM/CRLF handling, integrity checks

Commands:
- /privacy logging flatfile — switch to flat-file backend
- /history verify [<jid>] — check integrity of stored history

Fixes:
- Add missing 'off' guard in flatfile backend
- Enable CHLOG/HISTORY prefs when switching to flatfile mode

Logs stored in ~/.local/share/profanity/flatlog/{account}/{contact}/{date}.log
Format: {ISO8601} [{type}|{enc}|id:{id}|corrects:{id}] {sender}: {message}

Updated: CHANGELOG, CONTRIBUTING.md, profrc.example, man page, cmd_defs,
Makefile.am, test stubs, functional test support (PROF_FLATFILE=1)

tests/functionaltests/proftest.c

@@ -354,7 +354,8 @@ init_prof_test(void **state)
 
     /* Pre-write profrc with UI/notification defaults to ensure consistent,
      * deterministic output across tests (no timestamps, no roster/occupants
-     * panels, low input-blocking delay for fast command processing). */
+     * panels, low input-blocking delay for fast command processing).
+     * If PROF_FLATFILE=1, also select the flat-file database backend. */
     char profrc_path[512];
     snprintf(profrc_path, sizeof(profrc_path),
              "%s/profanity/profrc", xdg_config_home);
@@ -376,6 +377,11 @@ init_prof_test(void **state)
                 "[notifications]\n"
                 "message=false\n"
                 "room=false\n");
+        const char *flatfile_env = getenv("PROF_FLATFILE");
+        if (flatfile_env && strcmp(flatfile_env, "1") == 0) {
+            fprintf(prc, "[logging]\ndblog=flatfile\n");
+            printf("[PROF_TEST] Wrote profrc with dblog=flatfile: %s\n", profrc_path);
+        }
         fclose(prc);
     }