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

src/grist_mcp/main.py

@@ -1,5 +1,6 @@
 """Main entry point for the MCP server with SSE transport."""
 
+import json
 import os
 import sys
 from typing import Any
@@ -8,7 +9,8 @@ import uvicorn
 from mcp.server.sse import SseServerTransport
 
 from grist_mcp.server import create_server
-from grist_mcp.auth import AuthError
+from grist_mcp.config import load_config
+from grist_mcp.auth import Authenticator, AuthError
 
 
 Scope = dict[str, Any]
@@ -16,6 +18,29 @@ Receive = Any
 Send = Any
 
 
+def _get_bearer_token(scope: Scope) -> str | None:
+    """Extract Bearer token from Authorization header."""
+    headers = dict(scope.get("headers", []))
+    auth_header = headers.get(b"authorization", b"").decode()
+    if auth_header.startswith("Bearer "):
+        return auth_header[7:]
+    return None
+
+
+async def send_error(send: Send, status: int, message: str) -> None:
+    """Send an HTTP error response."""
+    body = json.dumps({"error": message}).encode()
+    await send({
+        "type": "http.response.start",
+        "status": status,
+        "headers": [[b"content-type", b"application/json"]],
+    })
+    await send({
+        "type": "http.response.body",
+        "body": body,
+    })
+
+
 def create_app():
     """Create the ASGI application."""
     config_path = os.environ.get("CONFIG_PATH", "/app/config.yaml")
@@ -24,15 +49,27 @@ def create_app():
         print(f"Error: Config file not found at {config_path}", file=sys.stderr)
         sys.exit(1)
 
-    try:
-        server = create_server(config_path)
-    except AuthError as e:
-        print(f"Authentication error: {e}", file=sys.stderr)
-        sys.exit(1)
+    config = load_config(config_path)
+    auth = Authenticator(config)
 
     sse = SseServerTransport("/messages")
 
     async def handle_sse(scope: Scope, receive: Receive, send: Send) -> None:
+        # Extract and validate token from Authorization header
+        token = _get_bearer_token(scope)
+        if not token:
+            await send_error(send, 401, "Missing Authorization header")
+            return
+
+        try:
+            agent = auth.authenticate(token)
+        except AuthError as e:
+            await send_error(send, 401, str(e))
+            return
+
+        # Create a server instance for this authenticated connection
+        server = create_server(auth, agent)
+
         async with sse.connect_sse(scope, receive, send) as streams:
             await server.run(
                 streams[0], streams[1], server.create_initialization_options()

src/grist_mcp/server.py

@@ -1,13 +1,11 @@
 """MCP server setup and tool registration."""
 
 import json
-import os
 
 from mcp.server import Server
 from mcp.types import Tool, TextContent
 
-from grist_mcp.config import load_config
-from grist_mcp.auth import Authenticator, AuthError, Agent
+from grist_mcp.auth import Authenticator, Agent
 
 from grist_mcp.tools.discovery import list_documents as _list_documents
 from grist_mcp.tools.read import list_tables as _list_tables
@@ -23,27 +21,18 @@ from grist_mcp.tools.schema import modify_column as _modify_column
 from grist_mcp.tools.schema import delete_column as _delete_column
 
 
-def create_server(config_path: str, token: str | None = None) -> Server:
-    """Create and configure the MCP server.
+def create_server(auth: Authenticator, agent: Agent) -> Server:
+    """Create and configure the MCP server for an authenticated agent.
 
     Args:
-        config_path: Path to the configuration YAML file.
-        token: Agent token for authentication. If not provided, reads from
-               GRIST_MCP_TOKEN environment variable.
+        auth: Authenticator instance for permission checks.
+        agent: The authenticated agent for this server instance.
 
-    Raises:
-        AuthError: If token is invalid or not provided.
+    Returns:
+        Configured MCP Server instance.
     """
-    config = load_config(config_path)
-    auth = Authenticator(config)
     server = Server("grist-mcp")
-
-    # Authenticate agent from token (required for all tool calls)
-    auth_token = token or os.environ.get("GRIST_MCP_TOKEN")
-    if not auth_token:
-        raise AuthError("No token provided. Set GRIST_MCP_TOKEN environment variable.")
-
-    _current_agent: Agent = auth.authenticate(auth_token)
+    _current_agent = agent
 
     @server.list_tools()
     async def list_tools() -> list[Tool]: