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/unittests/database/stub_database.c

@@ -25,6 +25,8 @@
 
 #include "database.h"
 
+db_backend_t* active_db_backend = NULL;
+
 gboolean
 log_database_init(ProfAccount* account)
 {
@@ -50,3 +52,27 @@ void
 log_database_close(void)
 {
 }
+GSList*
+log_database_verify_integrity(const gchar* const contact_barejid)
+{
+    return NULL;
+}
+void
+integrity_issue_free(integrity_issue_t* issue)
+{
+    if (issue) {
+        g_free(issue->file);
+        g_free(issue->message);
+        g_free(issue);
+    }
+}
+db_backend_t*
+db_backend_sqlite(void)
+{
+    return NULL;
+}
+db_backend_t*
+db_backend_flatfile(void)
+{
+    return NULL;
+}