feat(context): implement Redis-based caching layer (#84)

Phase 6 of Context Management Engine - Caching Layer:

- Add ContextCache with Redis integration
- Support fingerprint-based assembled context caching
- Support token count caching (model-specific)
- Support score caching (scorer + context + query)
- Add in-memory fallback with LRU eviction
- Add cache invalidation with pattern matching
- Add cache statistics reporting

Key features:
- Hierarchical cache key structure (ctx:type:hash)
- Automatic TTL expiration
- Memory cache for fast repeated access
- Graceful degradation when Redis unavailable

Tests: 29 new tests, 285 total context tests passing

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

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

backend/app/services/context/__init__.py

@@ -85,6 +85,9 @@ from .adapters import (
     OpenAIAdapter,
 )
 
+# Cache
+from .cache import ContextCache
+
 # Prioritization
 from .prioritization import (
     ContextRanker,
@@ -132,6 +135,8 @@ __all__ = [
     "BudgetAllocator",
     "TokenBudget",
     "TokenCalculator",
+    # Cache
+    "ContextCache",
     # Compression
     "ContextCompressor",
     "TruncationResult",