fix(contacts): sync device↔DB contacts, restore contact cache

- get_contacts_with_last_seen() reads from mc.contacts (device firmware)
  instead of DB, so /api/contacts/detailed returns only device contacts
- _sync_contacts_to_db() now bidirectional: downgrades stale 'device'
  contacts to 'advert' (cache-only) when not on device anymore
- delete_contact() sets source='advert' (cache) instead of 'deleted',
  keeping contacts visible in @mentions and cache filter
- get_contacts() returns all contacts (no 'deleted' filter needed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/database.py

@@ -116,7 +116,7 @@ class Database:
     def get_contacts(self) -> List[Dict]:
         with self._connect() as conn:
             rows = conn.execute(
-                "SELECT * FROM contacts WHERE source != 'deleted' ORDER BY last_seen DESC"
+                "SELECT * FROM contacts ORDER BY last_seen DESC"
             ).fetchall()
             return [dict(r) for r in rows]
 
@@ -147,18 +147,33 @@ class Database:
             return dict(row) if row else None
 
     def delete_contact(self, public_key: str) -> bool:
-        """Soft-delete: mark as 'deleted' instead of removing.
+        """Move contact to cache (source='advert') instead of deleting.
 
-        Keeps the row so FK references in direct_messages stay intact.
-        upsert_contact() overwrites source on re-add, auto-undeleting.
+        Contact stays visible in cache and @mentions but not in device list.
+        upsert_contact() overwrites source on re-add (back to 'device').
         """
         with self._connect() as conn:
             cursor = conn.execute(
-                "UPDATE contacts SET source = 'deleted', lastmod = datetime('now') WHERE public_key = ?",
+                "UPDATE contacts SET source = 'advert', lastmod = datetime('now') WHERE public_key = ?",
                 (public_key.lower(),)
             )
             return cursor.rowcount > 0
 
+    def downgrade_stale_device_contacts(self, active_device_keys: set) -> int:
+        """Downgrade contacts marked 'device' that are no longer on the device."""
+        with self._connect() as conn:
+            all_device = conn.execute(
+                "SELECT public_key FROM contacts WHERE source = 'device'"
+            ).fetchall()
+            stale_keys = [r['public_key'] for r in all_device
+                          if r['public_key'] not in active_device_keys]
+            if stale_keys:
+                conn.executemany(
+                    "UPDATE contacts SET source = 'advert', lastmod = datetime('now') WHERE public_key = ?",
+                    [(k,) for k in stale_keys]
+                )
+            return len(stale_keys)
+
     def set_contact_protected(self, public_key: str, protected: bool) -> bool:
         with self._connect() as conn:
             cursor = conn.execute(