Let agents drive the flow API over MCP

The engine now speaks MCP at /mcp, with a built-in OAuth 2.1 authorization
server in front of it: an agent registers itself, sends a human to the browser
to approve it, and exchanges the resulting code for a token. PKCE is required,
codes are single-use and stored only as hashes, the browser is redirected to
the URI that was registered rather than the one asked for, and refresh tokens
rotate so that replaying a spent one revokes the whole line.

Twenty tools cover reading, building, publishing and running flows, and each
one calls the same REST endpoint the dashboard calls, in-process, carrying the
caller's own token. That keeps one description of what a flow is and what may
be done to it — validation, the draft/publish split, the version check — and
means an agent can do nothing a person could not do in the browser.

Agent tokens are RS256 with a keypair of their own rather than the secret that
signs browser sessions, so deleting the key withdraws every agent without
logging anyone out, and deps.decode_token grew the branch that trusting a
second issuer will need when the hosted login arrives.

docker/compose.yml

@@ -145,6 +145,11 @@ services:
       # Flows are files in a git repository; secrets sit encrypted beside it.
       - FLOWS_DIR=/data/flows
       - SECRETS_FILE=/data/secrets.enc
+      # The MCP endpoint for agents, and the key its tokens are signed with.
+      # The key lives on the same volume as the flows, so it survives a rebuild
+      # and every issued token with it.
+      - MCP_ENABLED=${MCP_ENABLED-false}
+      - OAUTH_PRIVATE_KEY_FILE=/data/oauth-key.pem
 
     volumes:
       - app-flow-data:/data