feat: forward albums as a single grouped batch

[awdr74100]

Summary

Telegram delivers an album (media group) as separate messages sharing a groupedId, so each part was forwarded individually and reached the target as loose messages. This PR enables mtcute's messageGroupingInterval so a whole album arrives as one message_group update, and forwards the unit in a single forwardMessages call that keeps the grouped layout at the target.

Changes

• src/client.ts — enable updates.messageGroupingInterval: 250 (upstream-recommended value). Singles are unaffected; an album waits at most 250 ms before forwarding.
• src/forwarder.ts — route onNewMessage and onMessageGroup through one handler. With grouping enabled, album messages are dispatched only as message_group, so both subscriptions are required to see every message. Matching is wrapped so an unexpected error is logged with message ids instead of silently dropping the unit.
• src/filter.ts — matchesKeywords now evaluates a whole unit: an album's caption lives on one message but speaks for the entire post (include satisfied by any part; exclude on any part drops the unit).
• Content types stay per-message — the matching subset is still batched and reaches the target as one (smaller) album.
• Deletion-race salvage — when an album member is deleted between matching and sending, the batched forward fails with MESSAGE_ID_INVALID for the whole album; the forwarder now re-fetches the batch and retries with the surviving parts instead of losing them. isDeletedMessageError is exported from queue.ts so both layers share one classification.

Performance / behavior notes

• A 5-part album previously took 5 rate-limited sends (~500 ms+ of spacing, 5 API calls); it is now 1 call — faster overall and lower FloodWait exposure.
• If an album's parts arrive slower than the buffer window, it splits into two smaller albums at the target — cosmetic, never a lost message.
• catchUp was deliberately not enabled: mtcute documents it can replay duplicate updates after an unclean shutdown, i.e. double-forwards.

Testing

• 13 new tests: batched album forwarding, content-type subset, unit-level keyword semantics, deletion-race salvage (partial and total), single-message behavior unchanged, dry-run, and a guard that both dispatcher handlers are registered.
• pnpm typecheck / pnpm lint / pnpm test (75 passed) / pnpm build / pnpm knip / pnpm format:check all green.
• Verified live: albums from a monitored channel arrive at the target as a single grouped album.