feat: add pytest unit tests and CI workflow (#38)

* feat: add pytest unit tests and CI workflow

Add 67 tests covering both create_listing.py and dedup.py with shared
Pillow-based image fixtures. Add GitHub Actions workflow to run tests
on Python file changes.

* fix: address PR review feedback

- Use monkeypatch.chdir(tmp_path) so tests write to temp dirs instead
  of polluting the repo's README.md and index.html
- Strengthen unicode filename test to assert URL-encoded form (%C3%A9)
- Move hashlib import to module level in test_dedup.py
- Remove unused _zero_hash helper and Path import
- Prefix unused tuple unpacking variables with underscore

* fix: add docstrings and strengthen degenerate hash test

- Add docstrings to all test classes, methods, and helper functions
  to achieve 100% docstring coverage
- Strengthen test_skips_degenerate_hashes to assert groups == []
  instead of only checking for no-crash

* fix: use hardcoded MD5 digests and add fixture validation

- Replace hashlib.md5() calls with known digest constants to remove
  hashlib import from test module
- Add input validation to _make_gif fixture for clear error messages
  on empty colors or mismatched durations length

.github/workflows/test.yml

@@ -0,0 +1,25 @@
+name: Tests
+
+on:
+  push:
+    paths:
+      - '**.py'
+      - 'pyproject.toml'
+      - 'uv.lock'
+  pull_request:
+    paths:
+      - '**.py'
+      - 'pyproject.toml'
+      - 'uv.lock'
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+      - uses: astral-sh/setup-uv@5a095e7a2014a4212f075830d4f7277575a9d098 # v7
+      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6
+        with:
+          python-version: '3.14'
+      - run: uv sync --dev
+      - run: uv run pytest -v