perf: O(1) dedup/LMC cache, g_slist_append → prepend+reverse

- Add archive_ids hash set and stanza_senders hash map to
  ff_contact_state_t, populated during index build/extend via
  lightweight _ff_cache_line_ids() (no full parse overhead).
- Replace O(n) file scans in _ff_has_archive_id() and
  _ff_find_original_sender() with O(1) hash table lookups.
- Replace g_slist_append with g_slist_prepend + g_slist_reverse
  in _ff_apply_lmc, _flatfile_get_previous_chat, and
  ff_verify_integrity to avoid O(n²) list building.
- Add db_sqlite_last_changes() declaration to database.h.

src/database_flatfile.h

@@ -94,6 +94,8 @@ typedef struct
     ff_file_stamp_t stamp;
     off_t cursor_offset;
     int bom_len;
+    GHashTable* archive_ids;    // set: archive_id -> NULL (MAM dedup, O(1))
+    GHashTable* stanza_senders; // map: stanza_id -> from_jid (LMC validation, O(1))
 } ff_contact_state_t;
 
 // State management