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)
