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.

backend/tests/__init__.py

@@ -9,3 +9,7 @@ package — keeps a test run from touching the development data.
 import os
 
 os.environ["POSTGRES_DB"] = "app_test"
+# The MCP session manager can only be entered once per instance, and the suite
+# builds a TestClient — and so a lifespan — per test module. Tests that want the
+# endpoint mount it themselves.
+os.environ["MCP_ENABLED"] = "false"