Add birthday calendar backend support

Birthday calendars are ordinary local calendars flagged is_birthday, so they
flow to all clients via the merge read and inherit sharing/colors/reminders.

- models: LocalCalendar.is_birthday + birthday_notify_days_before;
  LocalEvent.external_uid (Contacts dedup) + birth_year
- build_local_event_dict: server-computed display_title "Name (age)" per
  occurrence, is_birthday flag for the client cake icon, and a reminder injected
  from birthday_notify_days_before so mobile schedulers fire it
- groups combined view keeps the birthday display_title instead of overwriting it
- local_router: calendar flags + event fields on create/update, plus
  GET /calendars/{id}/birthdays for importer reconcile by external_uid
- additive SQLite migrations

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -259,6 +259,29 @@ def _migrate():
         except Exception:
             pass
 
+        # Birthday calendars: flag + per-calendar "notify N days before".
+        for col, ddl in (
+            ("is_birthday", "ALTER TABLE local_calendars ADD COLUMN is_birthday BOOLEAN DEFAULT 0"),
+            ("birthday_notify_days_before", "ALTER TABLE local_calendars ADD COLUMN birthday_notify_days_before INTEGER"),
+        ):
+            try:
+                conn.execute(text(ddl))
+                conn.commit()
+                logging.info("Migration: added %s to local_calendars", col)
+            except Exception:
+                pass
+        # Birthday events: stable external id (Contacts dedup) + birth year.
+        for col, ddl in (
+            ("external_uid", "ALTER TABLE local_events ADD COLUMN external_uid VARCHAR(255)"),
+            ("birth_year", "ALTER TABLE local_events ADD COLUMN birth_year INTEGER"),
+        ):
+            try:
+                conn.execute(text(ddl))
+                conn.commit()
+                logging.info("Migration: added %s to local_events", col)
+            except Exception:
+                pass
+
 _migrate()
 
 app = FastAPI(title="Calendarr", docs_url=None, redoc_url=None)