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.lock

@@ -1148,6 +1148,7 @@ dependencies = [
  "criterion",
  "env_logger",
  "futures",
+ "http",
  "http-body-util",
  "hyper",
  "hyper-util",
@@ -1163,6 +1164,7 @@ dependencies = [
  "tokio",
  "tokio-rustls",
  "toml",
+ "tower",
 ]
 
 [[package]]