docs: add Google OAuth setup guide and update README/CLAUDE.md

- Create docs/GOOGLE_OAUTH.md: complete OAuth setup walkthrough
  (Google Cloud Console, env vars, JWT key generation, troubleshooting)
- README.md: add Google OAuth section with quick-setup steps + link
- CLAUDE.md: add JWT key generation tip + link to full guide

CLAUDE.md

@@ -78,8 +78,15 @@ the MCP server requires Google login before any tool call.
 2. Authorized redirect URIs: `<UNRAID_MCP_BASE_URL>/auth/callback`
 3. Copy Client ID + Secret to `~/.unraid-mcp/.env`
 
+**Generate a stable JWT signing key:**
+```bash
+python3 -c "import secrets; print(secrets.token_hex(32))"
+```
+
 **Omit `GOOGLE_CLIENT_ID` to run without auth** (default — preserves existing behaviour).
 
+**Full guide:** [`docs/GOOGLE_OAUTH.md`](docs/GOOGLE_OAUTH.md)
+
 ## Architecture
 
 ### Core Components