refactor: per-connection auth via Authorization header

Replace startup token authentication with per-SSE-connection auth.
Each client now passes Bearer token in Authorization header when
connecting. Server validates against config.yaml tokens and creates
isolated Server instance per connection.

- server.py: accept (auth, agent) instead of (config_path, token)
- main.py: extract Bearer token, authenticate, create server per connection
- Remove GRIST_MCP_TOKEN from docker-compose environments

deploy/dev/.env.example

@@ -1,3 +1 @@
-PORT=3000
-GRIST_MCP_TOKEN=your-token-here
-CONFIG_PATH=/app/config.yaml
+PORT=3010