feat(embed): add the Python router (FastAPI)

[mattj-monad]

Layer 5 of the Monad Embed SDK staged rollout (PRO-404) — the last router. Stacked on #15 (PRO-403) — targets feature/pro-403; review/merge the stack bottom-up.

What this adds

• routers/python — the Python port: embed_router(EmbedConfig(…)) as a mountable FastAPI/Starlette APIRouter (package monad-embed, httpx-based). Idiomatic Python; same contract.
• Adds the python leg to the conformance matrix: the harness venv also installs fastapi/uvicorn + the router (editable), boots servers/py_server.py, and runs ROUTER=python. Restores py_server.py (deferred from PRO-402).

With this, all three routers — ts, go, python — are proven against one contract.

Verification (local)

• python router unit tests: 11 passed
• conformance: ROUTER=python 14/14, ROUTER=go 14/14, ROUTER=ts 14/14

Stack

400 → 401 → 402 → 403 → 404 → 405 (docs/CI). Last router; 405 is the optional docs + CI-hardening finish.

🤖 Generated with Claude Code

https://claude.ai/code/session_011ASKA2VrufrNLGGrcFjY7n

[mattj-monad]

Force-pushed review fixes (Python router):

• Path injection: urllib.parse.quote(..., safe='') on every id + org (client.py and the router.py delete paths).
• Upstream body leak: MonadError now carries the detail for a server-side log; the browser gets a generic 502. The 500 and 401-resolver paths no longer echo str(exc).
  pytest 11 + ROUTER=python conformance 17/17.

[mattj-monad]

Force-pushed: 404/409 handling for the Python router — the MonadError handler now maps exc.status: 404 → not_found / 409 → conflict / else 502. ROUTER=python conformance 19/19.

[mattj-monad]

Force-pushed: fixed a null-handling bug found running the demo against live staging. When a tenant has no connectors of a kind, Monad returns { <kind>s: null } (not []); Python's .get(key, []) returns that null → 'NoneType' object is not iterable. TS (?? []) and Go (nil-slice) both coalesce null→[]; Python now does too via .get(key) or []. ROUTER=python 19/19.