fix(net): blob route cap, root_path/trailing-slash cap matching, throttled baseline-skip log

[wikithoughts]

Follow-ups from the PR #50 review (claude-review-pw-50.md, verdict MERGE-WITH-FOLLOWUP). This change touches only the cap table, _body_cap_for, the baseline-skip log, the README cap lines and tests/test_coordinator_limits.py.

What changed

• Blob upload route cap (Medium 1).
  • POST /jobs/{id}/blobs/{hash} now has its own cap: _BLOB_BODY_CAP = _BLOB_CAP + 16 KiB, which is 528 KiB. It was the 4 MiB ceiling.
  • The cap is derived from the handler's constant. _BLOB_CAP moved up next to the other caps and is defined only once.
  • The upload body is the raw chunk (application/octet-stream, operator.py:197-202), with no JSON or multipart wrapper. So the extra 16 KiB is only slack: at the boundary, put_blob's own exact 512 KiB check still decides, and it still answers chunk too large.
• Cap matching uses the app-relative path (Medium-Low 3).
  • _body_cap_for(method, path, root_path) removes a whole-segment root_path prefix before matching, the same way Starlette's get_route_path does. uvicorn 0.54 builds scope["path"] as root_path + path.
  • It also strips trailing slashes. So /pw/users and /users/ now get the 64 KiB cap for /users instead of falling back to the ceiling.
  • The middleware passes scope["root_path"] through.
• Baseline-skip log throttle (Low 5).
  • The [baseline] skipped … line is printed for the 1st skip and then every 100th (_BASELINE_SKIP_LOG_EVERY).
  • The /metrics baseline_skipped_busy counter still counts every skip exactly.
• README: blob uploads are no longer described as "at the ceiling". It also notes that a trailing slash or --root-path gets the same cap as the plain route.
• Tests (Medium 2):
  • Exact cap passes and cap+1 gets 413 on /users, on /tasks/{id}/result (4 MiB, spy shows the endpoint ran), on /jobs (4 MiB) and on blobs (512 KiB handler edge plus 528 KiB pre-auth route edge).
  • GET with a body over 64 KiB gets 413.
  • Trailing-slash and root_path-prefixed paths get the route cap.
  • The log is throttled and the counter stays exact.

Fails on main where behaviour changed

I ran the new test file against origin/main's coordinator_app.py: 6 failed, 26 passed. The failing tests:

• test_blob_route_cap_is_the_chunk_cap_plus_slack_not_the_ceiling. On main it errors on the missing _BLOB_BODY_CAP; without the new route cap, a 528 KiB+1 blob would reach auth and get 401.
• test_trailing_slash_gets_the_route_cap_not_the_ceiling. On main the request gets a 307 instead of a 413.
• test_root_path_prefix_gets_the_route_cap
• test_skip_log_is_throttled_but_the_counter_stays_exact
• test_busy_slots_skip_the_baseline_with_a_log_line_and_a_counter
• test_body_cap_for_routes_and_the_ceiling

The exact-cap tests for /users, /result and /jobs and the GET test pass on main too. That is expected: they cover behaviour that was already right but untested.

Verification (rebased on 003a0a4, head acc1f27)

• ruff check . && pytest tests/ -q --cov=passiveworkers --cov-fail-under=75 → exit 0: All checks passed!, 807 passed, 1 skipped, total coverage 83.64%.
• python -m py_compile $(git ls-files '*.py') → exit 0.
• pyright (now blocking for passiveworkers/net since ci: make pyright blocking, pin actions to commit SHAs, add CodeQL #51; installed into the local venv) → exit 0, 0 errors, 0 warnings, 0 informations.
• There is no UI change, so I did no browser check.

Not verified

• I did not run a real uvicorn --root-path server. The root_path test copies uvicorn 0.54's root_path + path scope using Starlette's TestClient (root_path="/pw").
• CI was not watched (the lane rules forbid it).
• The /jobs and /result ceilings are unchanged. The review's suggestion of about 256 KiB for /jobs was not part of this brief.

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