perf: one grouped query for latest per-flat application, not N

_wohnungen_context was calling last_application_for_flat(uid, f.id)
inside the flats loop — at 100 flats that's 101 queries per poll, and
with running applications the page polls every 3s. New
db.latest_applications_by_flat(user_id) returns {flat_id: row} via a
single grouped self-join.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/db.py

@@ -675,6 +675,24 @@ def last_application_for_flat(user_id: int, flat_id: str) -> Optional[sqlite3.Ro
     ).fetchone()
 
 
+def latest_applications_by_flat(user_id: int) -> dict:
+    """Return a dict {flat_id: latest-application-row} for all flats the user
+    has interacted with. One query instead of N."""
+    rows = _get_conn().execute(
+        """SELECT a.*
+             FROM applications a
+             JOIN (
+                 SELECT flat_id, MAX(started_at) AS maxstart
+                   FROM applications
+                  WHERE user_id = ?
+                  GROUP BY flat_id
+             ) m ON m.flat_id = a.flat_id AND m.maxstart = a.started_at
+            WHERE a.user_id = ?""",
+        (user_id, user_id),
+    ).fetchall()
+    return {r["flat_id"]: r for r in rows}
+
+
 def has_running_application(user_id: int) -> bool:
     row = _get_conn().execute(
         "SELECT 1 FROM applications WHERE user_id = ? AND finished_at IS NULL LIMIT 1",