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>

README.md

@@ -4,7 +4,7 @@ A full-stack application for managing Plex playlists with a FastAPI backend and
 
 ## Architecture
 
-- **Backend**: Python 3.13 + FastAPI + uv + ruff
+- **Backend**: Python 3.14 + FastAPI + uv + ruff
 - **Frontend**: TypeScript + Vue.js + Vite
 - **Database**: PostgreSQL 16
 - **Containerization**: Docker + Docker Compose
@@ -24,7 +24,7 @@ This project uses comprehensive linting and formatting:
 **Backend (Python):**
 - `ruff` - Fast Python linter and formatter
 - `pyright` - Type checking
-- `darglint` - Docstring linting (Google style)
+- `pydoclint` - Docstring linting (Google style)
 
 **Frontend (TypeScript/Vue):**
 - `eslint` - Linting with Vue and TypeScript support
@@ -131,6 +131,16 @@ plex-playlist/
 └── README.md
 ```
 
+## CI Operations
+
+For Gitea Actions runner image mirror maintenance, use:
+
+- `source scripts/gitea-actions/repair_runner_mirror.xsh`
+- `source scripts/gitea-actions/check_runner_images.xsh`
+- `source scripts/gitea-actions/collect_runner_diagnostics.xsh [since] [trace_id]`
+
+For full troubleshooting context, see `docs/GITEA_ACTIONS_TROUBLESHOOTING.md`.
+
 ## Environment Variables
 
 ### Backend
@@ -165,39 +175,41 @@ When running, the FastAPI automatic documentation is available at:
 
 # Manual Setup (if not using Docker)
 
-## Backend Setup (FastAPI, Python 3.13, uv, ruff, pyright)
+## Backend Setup (FastAPI, Python 3.14, uv, ruff, pyright)
 
 ### 1. Create and activate the uv virtual environment
 
 ```sh
-uv venv plex-playlist
-source plex-playlist/bin/activate
+cd backend
+uv venv .venv
+source .venv/bin/activate
 ```
 
 ### 2. Install dependencies
 
 ```sh
-uv pip install fastapi uvicorn[standard]
-uv pip install ruff
+uv sync --all-groups
 ```
 
 ### 3. Install dev tools
 
 ```sh
-uv pip install pyright
+uv run poe format
+uv run poe lint
+uv run poe test
 ```
 
 ### 4. Project structure
 
-- `app/` - FastAPI application code
+- `src/backend/` - FastAPI application code
 - `tests/` - Test suite
 - `pyrightconfig.json` - Pyright type checking config
-- `ruff.toml` - Ruff linter config
+- `pyproject.toml` - Ruff, pytest, and task configuration
 
 ### 5. Run the development server
 
 ```sh
-uvicorn app.main:app --reload
+uv run uvicorn backend.main:app --reload --app-dir src
 ```
 
 ---