test: add unit test infrastructure (Docker tester stage + CI)

- Add Dockerfile 'tester' stage (FROM builder):
  - Symlinks /usr/local/cuda/lib64/stubs/libcuda.so → libcuda.so.1
    so the test binary can satisfy the dynamic linker without a real GPU
  - Runs `cargo test --release` reusing the cached release build artifacts
    (no recompilation — tests complete in ~6s)
  - docker build --target tester . to run all 30 unit tests

- Add 'test' job to .gitea/workflows/docker-build.yml:
  - Runs before build-and-push (build-and-push needs: test)
  - Builds --target tester with registry build cache
  - Gate: build-and-push only runs when all tests pass

- Add run_tests.sh convenience script for local use:
  - Accepts optional test name filter as first argument
  - Respects CUDA_VERSION / UBUNTU_VERSION env overrides

All 30 unit tests pass:
  error::tests     — 7 tests (OOM detection, ModelNotReady HTTP shape)
  models::tests    — 17 tests (state machine, serialization, retry-after)
  worker::tests    — 6 tests (chunk ranges, silence snap/trim)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Dockerfile

@@ -82,6 +82,27 @@ RUN --mount=type=cache,target=/usr/local/cargo/registry \
     && cp target/release/whisper-server /usr/local/bin/whisper-server
 
 
+# ╔══════════════════════════════════════════════════════════╗
+# ║  STAGE 1b — tester                                       ║
+# ║  Runs unit tests against the release build artifacts     ║
+# ║  Uses CUDA stubs so tests run without a physical GPU     ║
+# ║                                                          ║
+# ║  Usage:                                                  ║
+# ║    docker build --target tester .                        ║
+# ╚══════════════════════════════════════════════════════════╝
+FROM builder AS tester
+
+# libcuda.so.1 stub — satisfies the dynamic linker without a real driver
+RUN ln -sf /usr/local/cuda/lib64/stubs/libcuda.so \
+           /usr/local/cuda/lib64/stubs/libcuda.so.1
+
+# Reuse the same cache mounts so no recompilation is needed
+RUN --mount=type=cache,target=/usr/local/cargo/registry \
+    --mount=type=cache,target=/build/target \
+    LD_LIBRARY_PATH=/usr/local/cuda/lib64/stubs \
+    cargo test --release
+
+
 # ╔══════════════════════════════════════════════════════════╗
 # ║  STAGE 2 — runtime                                       ║
 # ║  Minimal CUDA runtime image — no build tools             ║