perf(immich): skip full album fetch on idle ticks; delta-fetch for active ones

Optimizes polling for large Immich albums (tested path targets ~200k
assets). Combined impact on idle albums drops per-tick cost from ~150 MB
fetch to ~few hundred bytes; active albums fetch O(changes) instead of
O(library).

Core changes
- ImmichAlbumMeta + get_album_meta() using ?withoutAssets=true as a
  cheap change-detection probe.
- poll() fast-path: skip full fetch when meta fingerprint matches and
  no pending assets are outstanding.
- poll() delta-path: search/metadata with updatedAfter when fingerprint
  changed, falling back to full fetch on count decrease or mixed
  add+remove that delta can't reconcile.
- asyncio.gather over meta probes so a 20-album tracker pays one
  round-trip of latency instead of 20.
- Event payload cap (50 added / 200 removed) so a bulk import can't
  explode a Jinja template or exceed Telegram's message limits.
- Module-level users cache (1h TTL, sha256-keyed) shared across
  providers on the same Immich server.
- Tick-scoped shared-links cache via new
  get_all_shared_links_by_album() — one /api/shared-links request per
  tick instead of one per changed album.

Server changes
- meta_fingerprint JSON column on NotificationTrackerState + migration.
- watcher skips the asset_ids DB rewrite when the fingerprint didn't
  change, avoiding ~8 MB JSON writes on idle ticks for huge albums.
- Adaptive polling: after 10 empty ticks skip 1-in-2, after 30 skip
  1-in-4, reset on first detected change; resets on schedule changes.
- APScheduler jitter (interval/4, capped at 30s) to smooth thundering-
  herd bursts when many trackers share the same scan_interval.

packages/server/src/notify_bridge_server/database/migrations.py

@@ -309,6 +309,14 @@ async def migrate_schema(engine: AsyncEngine) -> None:
                     text(f"ALTER TABLE {state_table} ADD COLUMN shared INTEGER DEFAULT 0")
                 )
                 logger.info("Added shared column to %s table", state_table)
+            # meta_fingerprint — small JSON blob captured from the provider's
+            # cheap meta probe. An empty default means "unknown, do a full
+            # fetch next tick" so existing rows don't wrongly skip detection.
+            if not await _has_column(conn, state_table, "meta_fingerprint"):
+                await conn.execute(
+                    text(f"ALTER TABLE {state_table} ADD COLUMN meta_fingerprint TEXT DEFAULT '{{}}'")
+                )
+                logger.info("Added meta_fingerprint column to %s table", state_table)
 
         # Add language_code to telegram_chat if missing
         if await _has_table(conn, "telegram_chat"):