Backend runtime upgraded to Python 3.14 with exact dependency pinning (#57)

Signed-off-by: Cliff Hill <xlorep@darkhelm.org>

## Summary

Upgrades backend runtime baseline and dependency management for issue #10.

### Changes

1. **Python Baseline**: Updated from 3.13 to 3.14
   - Updated `backend/pyproject.toml` requires-python constraint
   - Updated `backend/pyrightconfig.json` pythonVersion
   - Updated all Dockerfile and CI references

2. **Dependency Pinning**: Switched to exact version pins in `backend/pyproject.toml`
   - All dev and runtime dependencies now use `==` instead of `>=`
   - `fastapi==0.120.2`, `uvicorn==0.38.0`
   - ruff, pyright, pytest suite pinned to current resolved versions
   - Regenerated `backend/uv.lock` under Python 3.14

3. **Startup Compatibility Guard** (TDD via RED→GREEN)
   - New `compatibility_status()` function evaluates runtime and pinned deps
   - Startup raises `RuntimeError` if policy fails
   - Implemented via FastAPI lifespan (non-deprecated) handler

4. **Compatibility Status Endpoint**
   - New `GET /compatibility` returns policy status, runtime version, and package checks
   - Shares single source of truth with startup validation

5. **Integration Tests**
   - Added failing-then-passing tests for startup guard and endpoint behavior
   - 100% coverage maintained

6. **Direnv Configuration**
   - Added `UV_PYTHON="3.14"` pin to repo `.envrc`
   - Ensures direnv creates/recreates venv with correct Python version

### Validation

- ✅ ruff format/check
- ✅ pyright strict (0 errors)
- ✅ pytest: 8 passed, 100% coverage (>=95 gate)
- ✅ pydoclint: pass
- ✅ xdoctest: pass

### Notes

- SQLAlchemy/SQLModel introduction deferred to next pass per scope
- Compatibility logic currently validates fastapi/uvicorn pins (runtime deps)
- Ready for container build validation and Renovate bot testing

Co-authored-by: copilotcoder <copilotcoder@darkhelm.org>
Reviewed-on: #57
Co-authored-by: Cliff Hill <xlorep@darkhelm.org>
Co-committed-by: Cliff Hill <xlorep@darkhelm.org>

backend/tests/integration/test_api.py

@@ -1,9 +1,11 @@
 """Integration tests for API endpoints."""
 
+from importlib import metadata
+
 import pytest
 from fastapi.testclient import TestClient
 
-from backend.main import app
+from backend.main import app, compatibility_status
 
 client = TestClient(app)
 
@@ -23,3 +25,68 @@ class TestAPIIntegration:
         response = client.get("/")
         assert response.status_code == 200
         assert response.json() == {"message": "Plex Playlist Backend API"}
+
+    def test_startup_rejects_invalid_runtime_policy(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Startup should fail fast when compatibility policy is invalid."""
+        monkeypatch.setenv("BACKEND_REQUIRED_PYTHON", "99.0")
+
+        with pytest.raises(RuntimeError), TestClient(app):
+            pass
+
+        monkeypatch.delenv("BACKEND_REQUIRED_PYTHON", raising=False)
+
+    def test_compatibility_endpoint_reports_policy_status(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Compatibility endpoint should expose runtime and pinning policy status."""
+        monkeypatch.setenv("BACKEND_REQUIRED_PYTHON", "3.14")
+
+        with TestClient(app) as local_client:
+            response = local_client.get("/compatibility")
+
+        assert response.status_code == 200
+        payload = response.json()
+        assert payload["ok"] is True
+        assert payload["required_python"] == "3.14"
+        assert "current_python" in payload
+        assert payload["python_policy_valid"] is True
+        assert "required_packages" in payload
+        assert payload["required_packages"]["fastapi"] == "0.120.2"
+        assert payload["required_packages"]["uvicorn"] == "0.38.0"
+        assert payload["package_errors"] == {}
+
+        monkeypatch.delenv("BACKEND_REQUIRED_PYTHON", raising=False)
+
+    def test_startup_rejects_malformed_required_python(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Startup should fail fast when Python policy is not strict major.minor."""
+        monkeypatch.setenv("BACKEND_REQUIRED_PYTHON", "3.14.1")
+
+        with pytest.raises(RuntimeError), TestClient(app):
+            pass
+
+        monkeypatch.delenv("BACKEND_REQUIRED_PYTHON", raising=False)
+
+    def test_compatibility_status_handles_missing_package_metadata(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Compatibility checks should handle missing package metadata gracefully."""
+
+        def raise_not_found(_: str) -> str:
+            raise metadata.PackageNotFoundError("fake")
+
+        monkeypatch.setattr("backend.main._installed_version", raise_not_found)
+        status = compatibility_status()
+        package_checks = status["package_checks"]
+        package_errors = status["package_errors"]
+
+        assert status["ok"] is False
+        assert isinstance(package_checks, dict)
+        assert isinstance(package_errors, dict)
+        assert package_checks["fastapi"] is False
+        assert package_checks["uvicorn"] is False
+        assert "fastapi" in package_errors
+        assert "uvicorn" in package_errors