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>

plans/primary-plan.md

@@ -205,7 +205,7 @@ async def _execute_telegram_notification(self, ...):
 - Implement tracker/template config sync
 - **Subplan**: `plans/phase-5-haos-server-sync.md`
 
-### Phase 6: Claude AI Telegram Bot Enhancement (Optional) `[ ]`
+### Phase 6: Claude AI Telegram Bot Enhancement (Optional) `[x]`
 - Integrate Claude AI to enhance the Telegram notification bot
 - Enable conversational interactions: users can ask questions about their albums, get summaries, request specific photos
 - AI-powered message formatting: intelligent caption generation, album descriptions