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>

run_tests.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# run_tests.sh — Run the unit test suite inside Docker (no GPU required)
+#
+# Uses the `tester` Docker stage which:
+#   1. Builds the release binary (or reuses cached build)
+#   2. Symlinks the CUDA stubs so libcuda.so.1 is satisfied without a driver
+#   3. Runs `cargo test --release`
+#
+# Usage:
+#   ./run_tests.sh                  # run all unit tests
+#   ./run_tests.sh models           # run only tests matching "models"
+#   CUDA_VERSION=12.1.0 ./run_tests.sh
+
+set -euo pipefail
+
+CUDA_VERSION=${CUDA_VERSION:-12.4.1}
+UBUNTU_VERSION=${UBUNTU_VERSION:-22.04}
+TEST_FILTER=${1:-}
+
+echo "==> Building tester stage (CUDA ${CUDA_VERSION} / Ubuntu ${UBUNTU_VERSION})..."
+docker build \
+  --target tester \
+  --build-arg CUDA_VERSION="${CUDA_VERSION}" \
+  --build-arg UBUNTU_VERSION="${UBUNTU_VERSION}" \
+  --tag whisper-tester:local \
+  .
+
+if [[ -n "${TEST_FILTER}" ]]; then
+  echo "==> Running tests matching '${TEST_FILTER}'..."
+  docker run --rm \
+    -e LD_LIBRARY_PATH=/usr/local/cuda/lib64/stubs \
+    whisper-tester:local \
+    sh -c "cd /build && ln -sf /usr/local/cuda/lib64/stubs/libcuda.so /usr/local/cuda/lib64/stubs/libcuda.so.1 && LD_LIBRARY_PATH=/usr/local/cuda/lib64/stubs cargo test --release '${TEST_FILTER}'"
+else
+  echo "==> All tests ran during docker build (tester stage)."
+  echo "    Build succeeded — all tests passed."
+fi