Ratchet coverage floors up to match measured coverage

Frontend floors move to 96/96/91/92 (measured 97.27/97.27/92.87/93.78)
and the backend gate to 80 (measured 83.47). Kept 1-2pp below measured so
real regressions fail without false positives on incidental drift.

These are a ratchet: raised as coverage grows, never lowered to make a
gate pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RXMhc9nT6sNz1SL4j8dznL

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
 
       - name: Test with coverage
         # Measured ~81%; set threshold 1-2pp below to catch regressions.
-        run: uv run pytest --cov=app --cov-report=term-missing --cov-fail-under=75
+        run: uv run pytest --cov=app --cov-report=term-missing --cov-fail-under=80
 
   # ---------------------------------------------------------------------------
   # Frontend: ESLint + Vitest with 90% coverage gate