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/command/cmd_funcs.c

@@ -6787,6 +6787,42 @@ cmd_history(ProfWin* window, const char* const command, gchar** args)
         return TRUE;
     }
 
+    if (g_strcmp0(args[0], "export") == 0) {
+#ifdef HAVE_SQLITE
+        const gchar* contact_jid = args[1]; // may be NULL (export all)
+        if (contact_jid) {
+            cons_show("Exporting SQLite history for %s to flat-file...", contact_jid);
+        } else {
+            cons_show("Exporting all SQLite history to flat-file...");
+        }
+        int n = log_database_export_to_flatfile(contact_jid);
+        if (n >= 0) {
+            cons_show("Export complete: %d message(s) exported.", n);
+        }
+#else
+        cons_show_error("Export requires SQLite support. Rebuild with --with-sqlite.");
+#endif
+        return TRUE;
+    }
+
+    if (g_strcmp0(args[0], "import") == 0) {
+#ifdef HAVE_SQLITE
+        const gchar* contact_jid = args[1]; // may be NULL (import all)
+        if (contact_jid) {
+            cons_show("Importing flat-file history for %s into SQLite...", contact_jid);
+        } else {
+            cons_show("Importing all flat-file history into SQLite...");
+        }
+        int n = log_database_import_from_flatfile(contact_jid);
+        if (n >= 0) {
+            cons_show("Import complete: %d message(s) imported.", n);
+        }
+#else
+        cons_show_error("Import requires SQLite support. Rebuild with --with-sqlite.");
+#endif
+        return TRUE;
+    }
+
     _cmd_set_boolean_preference(args[0], "Chat history", PREF_HISTORY);
 
     // if set to on, set chlog (/logging chat on)