feat(draft): add /history export|import for SQLite<->flatfile migration

DRAFT — not yet tested end-to-end with a live XMPP session.

New commands:
  /history export [<jid>]  — copy messages from SQLite to flat-file
  /history import [<jid>]  — copy messages from flat-file to SQLite
Both merge with existing data; duplicates are skipped using stanza-id
or a SHA-256 fallback key (timestamp + sender + body prefix).

Implementation (src/database_export.c):
- Export: paginate SQLite via get_previous_chat, read existing flatfile
  for dedup, write merged result via ff_write_line + atomic rename
- Import: parse flatfile lines via ff_parse_line, build dedup set from
  SQLite, insert new messages via add_incoming (preserves original
  timestamps for both directions)
- List all contacts: db_sqlite_list_contacts() queries UNION of
  DISTINCT from_jid/to_jid; flatfile enumerates flatlog directories

Wiring:
- database.h: declare export/import functions + db_sqlite_list_contacts
- database_sqlite.c: add db_sqlite_list_contacts()
- cmd_defs.c: add export/import to /history synopsis and args
- cmd_funcs.c: add export/import handlers in cmd_history()
- cmd_ac.c: add 'export' and 'import' to history_ac
- Makefile.am: add database_export.c to core_sources
- stub_database.c: add stubs for test linking
- profanity.1: document export/import in man page

All code guarded with #ifdef HAVE_SQLITE — builds cleanly without it.

src/database.h

@@ -87,6 +87,7 @@ extern db_backend_t* active_db_backend;
 // Backend registry
 #ifdef HAVE_SQLITE
 db_backend_t* db_backend_sqlite(void);
+GSList* db_sqlite_list_contacts(void);
 #endif
 db_backend_t* db_backend_flatfile(void);
 
@@ -101,4 +102,10 @@ ProfMessage* log_database_get_limits_info(const gchar* const contact_barejid, gb
 void log_database_close(void);
 GSList* log_database_verify_integrity(const gchar* const contact_barejid);
 
+// Cross-backend export/import (requires HAVE_SQLITE)
+#ifdef HAVE_SQLITE
+int log_database_export_to_flatfile(const gchar* const contact_jid);
+int log_database_import_from_flatfile(const gchar* const contact_jid);
+#endif
+
 #endif // DATABASE_H