chore(context): apply linter fixes and sort imports (#86)

Phase 8 of Context Management Engine - Final Cleanup:

- Sort __all__ exports alphabetically
- Sort imports per isort conventions
- Fix minor linting issues

Final test results:
- 311 context management tests passing
- 2507 total backend tests passing
- 85% code coverage

Context Management Engine is complete with all 8 phases:
1. Foundation: Types, Config, Exceptions
2. Token Budget Management
3. Context Scoring & Ranking
4. Context Assembly Pipeline
5. Model Adapters (Claude, OpenAI)
6. Caching Layer (Redis + in-memory)
7. Main Engine & Integration
8. Testing & Documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/context/scoring/recency.py

@@ -6,11 +6,11 @@ More recent content gets higher scores.
 """
 
 import math
-from datetime import UTC, datetime, timedelta
+from datetime import UTC, datetime
 from typing import Any
 
-from .base import BaseScorer
 from ..types import BaseContext, ContextType
+from .base import BaseScorer
 
 
 class RecencyScorer(BaseScorer):