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

.env.example

@@ -46,3 +46,10 @@
 # Alternatively, configure via the /telegram/setup dashboard endpoint at runtime.
 # Requires: pip install ".[telegram]"
 # TELEGRAM_TOKEN=
+
+# ── Discord bot ──────────────────────────────────────────────────────────────
+# Bot token from https://discord.com/developers/applications
+# Alternatively, configure via the /discord/setup dashboard endpoint at runtime.
+# Requires: pip install ".[discord]"
+# Optional: pip install pyzbar Pillow  (for QR code invite detection from screenshots)
+# DISCORD_TOKEN=