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)

CONTRIBUTING.md

@@ -123,6 +123,20 @@ Test your changes with the following tools to find mistakes.
 
 Run `make check` to run the unit tests with your current configuration or `./ci-build.sh` to check with different switches passed to configure.
 
+### flat-file backend tests
+
+To run functional tests with the flat-file database backend (instead of SQLite):
+
+```bash
+make check-functional-flatfile
+```
+
+Or manually for a single group:
+
+```bash
+PROF_FLATFILE=1 PROF_TEST_GROUP=1 ./tests/functionaltests/functionaltests 1
+```
+
 ### valgrind
 We provide a suppressions file `prof.supp`. It is a combination of the suppressions for shipped with glib2, python and custom rules.