Add cron container and per-household timezone for auto-reminders

- docker-compose.yml: meal-cron service loops send_reminders_cron.py every 60s
- send_reminders_cron.py: checks each household's local time, sends ntfy
  reminders at 10:00 (lunch) and 16:00 (dinner), guarded against duplicates
- models.py: timezone, last_lunch_reminder, last_dinner_reminder on households
- meals.py: /admin/timezone route for admins to set household timezone
- dashboard: timezone selector (20 common zones) in Account Settings
- i18n: 5 new translation keys for timezone strings
- Dockerfile: copy send_reminders_cron.py
- tests: 4 new timezone tests (admin set, non-admin denied, default UTC, auth)
- docs: updated AGENTS.md

tests/test_ntfy.py

@@ -229,3 +229,65 @@ class TestNtfyTestEndpoint:
             "ntfy_url": "https://ntfy.sh/test",
         })
         assert resp.status_code == 302
+
+
+class TestTimezoneSettings:
+    def test_admin_can_set_timezone(self, admin_client):
+        resp = admin_client.post("/admin/timezone", data={
+            "timezone": "Europe/Zurich",
+        }, follow_redirects=True)
+        assert resp.status_code == 200
+
+        from models import get_db
+        db = get_db()
+        row = db.execute(
+            "SELECT timezone FROM households WHERE name = ?", ("Admin Household",)
+        ).fetchone()
+        assert row["timezone"] == "Europe/Zurich"
+
+    def test_non_admin_cannot_set_timezone(self, client):
+        # Register a regular user in an existing household
+        client.post("/register", data={
+            "username": "admin_tz",
+            "password": "admin_tz_pass",
+            "confirm": "admin_tz_pass",
+            "household_action": "create",
+            "new_household_name": "TZ Household",
+        }, follow_redirects=True)
+        client.get("/logout")
+
+        from models import get_household_by_name
+        hh = get_household_by_name("TZ Household")
+        client.post("/register", data={
+            "username": "regular_tz",
+            "password": "regular_tz_pass",
+            "confirm": "regular_tz_pass",
+            "household_action": "join",
+            "household_id": str(hh["id"]),
+        }, follow_redirects=True)
+
+        resp = client.post("/admin/timezone", data={
+            "timezone": "Europe/Zurich",
+        }, follow_redirects=True)
+        assert resp.status_code == 200
+        # Timezone should NOT have changed
+        from models import get_db
+        db = get_db()
+        row = db.execute(
+            "SELECT timezone FROM households WHERE id = ?", (hh["id"],)
+        ).fetchone()
+        assert row["timezone"] == "UTC"  # unchanged
+
+    def test_default_timezone_is_utc(self, admin_client):
+        from models import get_db
+        db = get_db()
+        row = db.execute(
+            "SELECT timezone FROM households WHERE name = ?", ("Admin Household",)
+        ).fetchone()
+        assert row["timezone"] == "UTC"
+
+    def test_timezone_requires_login(self, client):
+        resp = client.post("/admin/timezone", data={
+            "timezone": "Europe/Zurich",
+        })
+        assert resp.status_code == 302