Add pytest-cov for test coverage

- Add pytest-cov as dev dependency
- Configure coverage in pyproject.toml
- Exclude tests, __pycache__, seed.py from coverage
- Enable branch coverage
- Add .coverage to gitignore

.gitignore

@@ -28,3 +28,4 @@ next_pr.md
 
 # Notes
 notes/
+.coverage

backend/pyproject.toml

@@ -21,6 +21,7 @@ dev = [
     "aiosqlite>=0.20.0",
     "mypy>=1.13.0",
     "ruff>=0.14.10",
+    "pytest-cov>=7.0.0",
 ]
 
 [tool.mypy]
@@ -55,3 +56,15 @@ quote-style = "double"
 [tool.ruff.lint.per-file-ignores]
 "tests/*" = ["E501"]  # Allow longer lines in tests for readability
 
+[tool.coverage.run]
+source = ["."]
+omit = ["tests/*", "__pycache__/*", "seed.py"]
+branch = true
+
+[tool.coverage.report]
+exclude_lines = [
+    "pragma: no cover",
+    "if TYPE_CHECKING:",
+]
+show_missing = true
+