test: add comprehensive database tests and CI pipeline

Phase 6 test infrastructure (6.1, 6.2, 6.5):
- Fix .coveragerc source path (src -> .)
- Fix pre-existing test failure (patch target + method rename)
- Add 52 database layer tests covering CRUD, junction table tags,
  search, pagination, statistics, image linking, and edge cases
- Add GitHub Actions CI workflow (Python 3.10-3.12)
- Total: 61 tests, all passing

.github/workflows/test.yml

@@ -0,0 +1,30 @@
+name: Tests
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12"]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install Pillow
+
+      - name: Run tests
+        run: python -m unittest discover tests/ -v