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)

src/database.h

@@ -48,6 +48,45 @@ typedef enum {
     DB_RESPONSE_SUCCESS
 } db_history_result_t;
 
+// Integrity verification issue levels
+typedef enum {
+    INTEGRITY_ERROR,
+    INTEGRITY_WARNING,
+    INTEGRITY_INFO
+} integrity_level_t;
+
+// A single integrity issue found during verification
+typedef struct {
+    integrity_level_t level;
+    char* file;
+    int line;
+    char* message;
+} integrity_issue_t;
+
+void integrity_issue_free(integrity_issue_t* issue);
+
+// Backend vtable: pluggable storage backends implement this interface
+typedef struct db_backend_t {
+    const char* name;
+    gboolean (*init)(ProfAccount* account);
+    void (*close)(void);
+    void (*add_incoming)(ProfMessage* message);
+    void (*add_outgoing_chat)(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
+    void (*add_outgoing_muc)(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
+    void (*add_outgoing_muc_pm)(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
+    db_history_result_t (*get_previous_chat)(const gchar* const contact_barejid, const gchar* start_time, const gchar* end_time, gboolean from_start, gboolean flip, GSList** result);
+    ProfMessage* (*get_limits_info)(const gchar* const contact_barejid, gboolean is_last);
+    GSList* (*verify_integrity)(const gchar* const contact_barejid);
+} db_backend_t;
+
+// Active backend (set during init based on PREF_DBLOG)
+extern db_backend_t* active_db_backend;
+
+// Backend registry
+db_backend_t* db_backend_sqlite(void);
+db_backend_t* db_backend_flatfile(void);
+
+// Public API (dispatches to active_db_backend)
 gboolean log_database_init(ProfAccount* account);
 void log_database_add_incoming(ProfMessage* message);
 void log_database_add_outgoing_chat(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
@@ -56,5 +95,6 @@ void log_database_add_outgoing_muc_pm(const char* const id, const char* const ba
 db_history_result_t log_database_get_previous_chat(const gchar* const contact_barejid, const gchar* start_time, const gchar* end_time, gboolean from_start, gboolean flip, GSList** result);
 ProfMessage* log_database_get_limits_info(const gchar* const contact_barejid, gboolean is_last);
 void log_database_close(void);
+GSList* log_database_verify_integrity(const gchar* const contact_barejid);
 
 #endif // DATABASE_H