fix: declare ruff as a dev dependency

CI has no nix shell, and ruff was only ever a nix package, so both lint steps
died with "Failed to spawn: ruff". Pinned exactly: the formatter deciding what
is correct must be one version, or CI rejects what the dev shell produced.

backend/pyproject.toml

@@ -36,6 +36,7 @@ dev = [
     "pytest>=8.4.2",
     "pytest-asyncio>=1.2.0",
     "pytest-databases[postgres]>=0.15.0",
+    "ruff==0.15.14",
 ]
 
 [tool.ruff.lint.per-file-ignores]
@@ -48,4 +49,4 @@ asyncio_mode = "auto"
 testpaths = ["tests"]
 filterwarnings = [                                                                          
     "ignore::jwt.warnings.InsecureKeyLengthWarning",                                      
-] 
+]