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/pyproject.toml

@@ -4,23 +4,23 @@ requires = ["hatchling"]
 
 [dependency-groups]
 dev = [
-  "ruff>=0.6.0",
-  "pyright>=1.1.380",
-  "pydoclint>=0.3.8",
-  "pytest>=7.4.0",
-  "pytest-asyncio>=0.21.0",
-  "pytest-cov>=4.1.0",
-  "typeguard>=4.1.0",
-  "httpx>=0.25.0",  # For testing async HTTP calls
-  "pytest-mock>=3.12.0",
+  "ruff==0.14.1",
+  "pyright==1.1.406",
+  "pydoclint==0.8.3",
+  "pytest==8.4.2",
+  "pytest-asyncio==1.2.0",
+  "pytest-cov==7.0.0",
+  "typeguard==4.4.4",
+  "httpx==0.28.1",  # For testing async HTTP calls
+  "pytest-mock==3.15.1",
   # File format and linting tools
-  "pre-commit>=3.0.0",  # For running pre-commit hooks in CI
-  "pyyaml>=6.0",
-  "yamllint>=1.35.0",
-  "toml-sort>=0.23.0",
-  "language-formatters-pre-commit-hooks>=2.14.0",  # For pretty-format-toml
-  "xdoctest>=1.1.0",  # For doctest support
-  "poethepoet>=0.24.0"  # Task runner for unified development workflows
+  "pre-commit==4.3.0",  # For running pre-commit hooks in CI
+  "pyyaml==6.0.3",
+  "yamllint==1.37.1",
+  "toml-sort==0.24.3",
+  "language-formatters-pre-commit-hooks==2.15.0",  # For pretty-format-toml
+  "xdoctest==1.3.0",  # For doctest support
+  "poethepoet==0.41.0"  # Task runner for unified development workflows
 ]
 
 [project]
@@ -30,18 +30,18 @@ classifiers = [
   "Intended Audience :: Developers",
   "License :: OSI Approved :: MIT License",
   "Programming Language :: Python :: 3",
-  "Programming Language :: Python :: 3.13"
+  "Programming Language :: Python :: 3.14"
 ]
 dependencies = [
-  "fastapi>=0.100.0",
-  "uvicorn>=0.22.0"
+  "fastapi==0.120.2",
+  "uvicorn==0.38.0"
 ]
 description = "Backend service for Plex playlist management"
 keywords = ["plex", "playlist", "media", "management"]
 license = "MIT"
 name = "plex-playlist-backend"
 readme = "../README.md"
-requires-python = ">=3.13"
+requires-python = ">=3.14"
 version = "0.1.0"
 
 [tool.coverage]
@@ -133,9 +133,9 @@ lint-frontend = {shell = "cd frontend && yarn lint", help = "Lint frontend TypeS
 # === Parallel Execution for Speed ===
 lint-parallel = {shell = "poe lint-backend & poe lint-frontend & wait", help = "Run linting in parallel for speed"}
 # === CI/CD Tasks ===
-pre-commit-install = {shell = "pre-commit install", help = "Install pre-commit hooks"}
-pre-commit-run = {shell = "pre-commit run --all-files", help = "Run all pre-commit hooks"}
-pre-commit-update = {shell = "pre-commit autoupdate", help = "Update pre-commit hook versions"}
+pre-commit-install = {shell = "pre-commit install --config ../.pre-commit-config.yaml", help = "Install pre-commit hooks"}
+pre-commit-run = {shell = "pre-commit run --all-files --config ../.pre-commit-config.yaml", help = "Run all pre-commit hooks"}
+pre-commit-update = {shell = "pre-commit autoupdate --config ../.pre-commit-config.yaml", help = "Update pre-commit hook versions"}
 # === Quality Gates (Mimics CI Pipeline) ===
 quality-gate = [
   {shell = "echo '🔍 Running quality gate checks...'"},
@@ -212,7 +212,7 @@ testpaths = ["tests"]
 [tool.ruff]
 line-length = 88
 src = ["src"]
-target-version = "py313"
+target-version = "py314"
 
 [tool.ruff.lint]
 ignore = [