Add Claude AI Telegram bot enhancement (Phase 6)

Integrate Claude AI into the notification system for intelligent
conversational interactions and AI-powered captions.

New modules:
- ai/service.py: Claude API client with conversation history,
  caption generation, and album activity summarization
- ai/telegram_webhook.py: Telegram webhook handler for incoming
  bot messages, routes to AI service for responses

Features:
- Conversational bot: users chat with the bot about albums
- AI captions: intelligent notification messages based on album
  context (people, locations, dates) - enabled per target via
  "ai_captions" config flag
- Album summaries: "what's new?" triggers AI-generated overview
- /start command with welcome message
- Webhook register/unregister endpoints

Architecture:
- Per-chat conversation history (in-memory, capped at 20 messages)
- Graceful degradation: AI features completely disabled without
  IMMICH_WATCHER_ANTHROPIC_API_KEY env var (zero impact)
- AI caption failure falls back to Jinja2 template rendering
- Health endpoint reports ai_enabled status

Config: IMMICH_WATCHER_ANTHROPIC_API_KEY, IMMICH_WATCHER_AI_MODEL,
IMMICH_WATCHER_AI_MAX_TOKENS

Server now has 45 API routes (was 42 after Phase 5).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/server/src/immich_watcher_server/config.py

@@ -21,6 +21,11 @@ class Settings(BaseSettings):
     port: int = 8420
     debug: bool = False
 
+    # Claude AI (optional - leave empty to disable AI features)
+    anthropic_api_key: str = ""
+    ai_model: str = "claude-sonnet-4-20250514"
+    ai_max_tokens: int = 1024
+
     model_config = {"env_prefix": "IMMICH_WATCHER_"}
 
     @property