test: a size and complexity ratchet; coverage reported on both sides

The "class size" item from the quality checklist, measured:
- A shipped file over 800 lines may not grow past its line count today
  (13 frontend, 11 backend). No other file may cross 800.
- A function over the complexity limit may not rise, and no new one may
  appear. The limit is eslint `complexity` 20 on the frontend (64 functions
  over it today) and ruff C901 10 on the backend (32).
- Each is a per-file ceiling. Shrinking is free; an entry back under the
  limit must be deleted, so the lists only shrink.

C901 is measured in a test rather than switched on in ruff.toml: ruff has no
per-function baseline, and a noqa would let a function grow without limit.

Coverage (owner decision D4) is printed with no threshold:
`vitest --coverage` (v8) gives 80% of lines, and the pytest lane ends with
the total for app/, 78%. The bar stays "every regression test seen red".
.coverage is ignored by git and kept out of the image's build context.

backend/requirements-dev.txt

@@ -7,7 +7,10 @@
 # basedpyright: the backend type-check lane (E27 S3). Pinned like everything else here — a type
 # checker whose version floats silently changes what "clean" means, and the ratchet baseline it
 # writes is keyed to the diagnostics THIS version produces.
+# pytest-cov: the coverage REPORT the gate prints (E32 D4) — a number to read, deliberately with no
+# threshold; "every regression test seen red" is the bar, not a percentage.
 basedpyright==1.40.1
 pytest==8.4.2
+pytest-cov==7.1.0
 ruff==0.15.21
 httpx2==2.13.0