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/app.py

@@ -400,6 +400,9 @@ def _wohnungen_context(user) -> dict:
     age_cutoff = None
     if max_age_hours:
         age_cutoff = datetime.now(timezone.utc) - timedelta(hours=int(max_age_hours))
+    # One query for this user's latest application per flat, instead of a
+    # per-flat query inside the loop.
+    latest_apps = db.latest_applications_by_flat(uid)
     flats_view = []
     for f in flats:
         if f["id"] in rejected:
@@ -417,8 +420,7 @@ def _wohnungen_context(user) -> dict:
             "wbs": f["wbs"],
         }, filters):
             continue
-        last = db.last_application_for_flat(uid, f["id"])
+        flats_view.append({"row": f, "last": latest_apps.get(f["id"])})
-        flats_view.append({"row": f, "last": last})
 
     rejected_view = db.rejected_flats(uid)
     enrichment_counts = db.enrichment_counts()

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",