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>

.pre-commit-config.yaml

@@ -78,7 +78,7 @@ repos:
       # ESLint with auto-fix for pre-commit (CI uses --no-fix)
       - id: eslint
         name: eslint
-        entry: bash -c 'cd frontend && yarn eslint . --fix'
+        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && cd frontend && if [ "${CI:-}" = "true" ]; then corepack yarn eslint . --max-warnings=0; else corepack yarn eslint . --fix; fi'
         language: system
         files: ^frontend/.*\.(js|ts|vue)$
         pass_filenames: false
@@ -86,7 +86,7 @@ repos:
       # Prettier with auto-format for pre-commit (CI uses --check)
       - id: prettier
         name: prettier
-        entry: bash -c 'cd frontend && yarn prettier --write src/'
+        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && cd frontend && if [ "${CI:-}" = "true" ]; then corepack yarn prettier --check src/; else corepack yarn prettier --write src/; fi'
         language: system
         files: ^frontend/.*\.(js|ts|vue|json|css|scss|md)$
         pass_filenames: false
@@ -94,7 +94,7 @@ repos:
       # TypeScript type checking (same as CI)
       - id: typescript-check
         name: typescript-check
-        entry: bash -c 'cd frontend && yarn vue-tsc --noEmit'
+        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && cd frontend && corepack yarn vue-tsc --noEmit'
         language: system
         files: ^frontend/.*\.(ts|vue)$
         pass_filenames: false
@@ -102,7 +102,7 @@ repos:
       # TSDoc linting with auto-fix for pre-commit (CI uses --no-fix)
       - id: tsdoc-lint
         name: tsdoc-lint
-        entry: bash -c 'cd frontend && yarn eslint . --ext .ts,.vue --fix'
+        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && cd frontend && if command -v yarn >/dev/null 2>&1; then if [ "${CI:-}" = "true" ]; then yarn eslint . --ext .ts,.vue --max-warnings=0; else yarn eslint . --ext .ts,.vue --fix; fi; else if [ "${CI:-}" = "true" ]; then corepack yarn eslint . --ext .ts,.vue --max-warnings=0; else corepack yarn eslint . --ext .ts,.vue --fix; fi; fi'
         language: system
         files: ^frontend/.*\.(ts|vue)$
         pass_filenames: false