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_sqlite.c

@@ -817,3 +817,37 @@ db_backend_sqlite(void)
 {
     return &sqlite_backend;
 }
+
+GSList*
+db_sqlite_list_contacts(void)
+{
+    GSList* contacts = NULL;
+    if (!g_chatlog_database)
+        return NULL;
+
+    const Jid* myjid = connection_get_jid();
+    if (!myjid || !myjid->barejid)
+        return NULL;
+
+    const char* q = "SELECT DISTINCT `from_jid` FROM `ChatLogs` "
+                    "WHERE `from_jid` != ? "
+                    "UNION "
+                    "SELECT DISTINCT `to_jid` FROM `ChatLogs` "
+                    "WHERE `to_jid` != ?";
+    sqlite3_stmt* stmt = NULL;
+    if (!_db_prepare_ctx(q, &stmt, "db_sqlite_list_contacts"))
+        return NULL;
+
+    sqlite3_bind_text(stmt, 1, myjid->barejid, -1, SQLITE_STATIC);
+    sqlite3_bind_text(stmt, 2, myjid->barejid, -1, SQLITE_STATIC);
+
+    while (sqlite3_step(stmt) == SQLITE_ROW) {
+        const char* jid = (const char*)sqlite3_column_text(stmt, 0);
+        if (jid && strlen(jid) > 0) {
+            contacts = g_slist_append(contacts, g_strdup(jid));
+        }
+    }
+    sqlite3_finalize(stmt);
+
+    return contacts;
+}