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)

CHANGELOG

@@ -1,3 +1,16 @@
+0.16.0 (unreleased)
+===================
+
+Changes:
+- Add flat-file database backend as alternative to SQLite for message history.
+  Stores messages as human-readable plain text files. Configure with `/privacy logging flatfile`.
+  Files are stored in ~/.local/share/profanity/flatlog/.
+- Add `/history verify [<jid>]` command to check integrity of stored message
+  history (works with both SQLite and flat-file backends).
+- Add vtable-based database backend abstraction allowing pluggable storage.
+- Add `make check-functional-flatfile` target to run functional tests with
+  flat-file backend.
+
 0.15.0 (2025-03-27)
 ===================