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>

frontend/.prettierrc.json

@@ -4,5 +4,6 @@
   "singleQuote": true,
   "printWidth": 100,
   "tabWidth": 2,
-  "useTabs": false
+  "useTabs": false,
+  "endOfLine": "lf"
 }

frontend/package.json

@@ -38,7 +38,7 @@
     "eslint-plugin-tsdoc": "^0.3.0",
     "eslint-plugin-vue": "^9.28.0",
     "jsdom": "^23.0.0",
-    "prettier": "^3.0.0",
+    "prettier": "3.6.2",
     "typescript": "~5.3.0",
     "vite": "^7.1.10",
     "vitest": "^3.2.4",

frontend/yarn.lock

@@ -3280,7 +3280,7 @@ __metadata:
     eslint-plugin-tsdoc: "npm:^0.3.0"
     eslint-plugin-vue: "npm:^9.28.0"
     jsdom: "npm:^23.0.0"
-    prettier: "npm:^3.0.0"
+    prettier: "npm:3.6.2"
     typescript: "npm:~5.3.0"
     vite: "npm:^7.1.10"
     vitest: "npm:^3.2.4"
@@ -3319,7 +3319,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"prettier@npm:^3.0.0":
+"prettier@npm:3.6.2":
   version: 3.6.2
   resolution: "prettier@npm:3.6.2"
   bin: