feat: add Discord integration with chat_bridge abstraction layer

Introduces a vendor-agnostic chat platform architecture:

- chat_bridge/base.py: ChatPlatform ABC, ChatMessage, ChatThread
- chat_bridge/registry.py: PlatformRegistry singleton
- chat_bridge/invite_parser.py: QR + Ollama vision invite extraction
- chat_bridge/vendors/discord.py: DiscordVendor with native threads

Workflow: paste a screenshot of a Discord invite or QR code at
POST /discord/join → Timmy extracts the invite automatically.

Every Discord conversation gets its own thread, keeping channels clean.
Bot responds to @mentions and DMs, routes through Timmy agent.

43 new tests (base classes, registry, invite parser, vendor, routes).

https://claude.ai/code/session_01WU4h3cQQiouMwmgYmAgkMM

src/config.py

@@ -16,6 +16,9 @@ class Settings(BaseSettings):
     # Telegram bot token — set via TELEGRAM_TOKEN env var or the /telegram/setup endpoint
     telegram_token: str = ""
 
+    # Discord bot token — set via DISCORD_TOKEN env var or the /discord/setup endpoint
+    discord_token: str = ""
+
     # ── AirLLM / backend selection ───────────────────────────────────────────
     # "ollama"  — always use Ollama (default, safe everywhere)
     # "airllm"  — always use AirLLM (requires pip install ".[bigbrain]")