feat: API endpoint tests, coverage target

- 8 new axum handler tests: health, stats, query-log, overrides CRUD,
  cache, blocking stats, services CRUD, dashboard HTML
- Tests use tower::oneshot — no network, no server startup
- test_ctx() builds minimal ServerCtx for isolated testing
- `make coverage` target (cargo-tarpaulin), separate from `make all`
- 82 total tests (was 74)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -32,6 +32,8 @@ ring = "0.17"
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }
+tower = { version = "0.5", features = ["util"] }
+http = "1"
 
 [[bench]]
 name = "hot_path"