Add ruff linter/formatter for Python

- Add ruff as dev dependency
- Configure ruff in pyproject.toml with strict 88-char line limit
- Ignore B008 (FastAPI Depends pattern is standard)
- Allow longer lines in tests for readability
- Fix all lint issues in source files
- Add Makefile targets: lint-backend, format-backend, fix-backend

backend/pyproject.toml

@@ -20,6 +20,7 @@ dev = [
     "httpx>=0.28.1",
     "aiosqlite>=0.20.0",
     "mypy>=1.13.0",
+    "ruff>=0.14.10",
 ]
 
 [tool.mypy]
@@ -30,3 +31,27 @@ check_untyped_defs = true
 ignore_missing_imports = true
 exclude = ["tests/"]
 
+[tool.ruff]
+line-length = 88
+target-version = "py311"
+
+[tool.ruff.lint]
+select = [
+    "E",   # pycodestyle errors
+    "F",   # pyflakes
+    "I",   # isort
+    "B",   # flake8-bugbear
+    "UP",  # pyupgrade
+    "SIM", # flake8-simplify
+    "RUF", # ruff-specific rules
+]
+ignore = [
+    "B008",  # function-call-in-default-argument (standard FastAPI pattern with Depends)
+]
+
+[tool.ruff.format]
+quote-style = "double"
+
+[tool.ruff.lint.per-file-ignores]
+"tests/*" = ["E501"]  # Allow longer lines in tests for readability
+