feat(cache): smart caching strategy for dashboard & analytics

[lalalic]

Summary

Intelligent multi-layer caching system with smart invalidation for dashboard and analytics performance. Closes #127.

Architecture

Request → L1 Cache (Memory, ~1μs) → L2 Cache (Redis, ~1ms) → Database

L1 Cache (In-Memory)

• Python dict-based with configurable max size (1000 entries)
• LRU-like eviction with automatic expired entry cleanup
• Hit/miss tracking with real-time statistics

L2 Cache (Redis)

• Persistent across process restarts
• Shared across application instances
• Pattern-based key scanning for targeted invalidation

Features

Configurable TTL Policies (8 types)

Data Type	TTL	Use Case
dashboard	5 min	Dashboard widgets
analytics	10 min	Spending trends, charts
insights	15 min	AI-generated insights
categories	1 hour	Category lists
user_prefs	2 hours	User preferences
summary	30 min	Financial summaries
report	1 hour	Generated reports
default	5 min	Everything else

@cached Decorator

@cached(data_type="dashboard")
def get_dashboard(user_id):
    return expensive_computation()

Smart Invalidation

• Per-user invalidation across specific data types
• Global cache flush capability
• Automatic L1/L2 coordination

Cache Warming

Pre-populate caches for users via configurable warmer functions.

API Endpoints (8 total)

Method	Endpoint	Description
GET	/cache/stats	L1 + L2 statistics
POST	/cache/stats/reset	Reset counters
GET	/cache/policies	TTL policy listing
POST	/cache/invalidate/user	Per-user invalidation
POST	/cache/invalidate/all	Full cache flush
POST	/cache/warm	Pre-populate cache
POST	/cache/test	Test set/get/delete
GET	/cache/health	Health check

Testing

46 tests covering:

• L1 cache operations (9 tests): set/get/delete, eviction, stats, expiry
• Cache key generation (5 tests): determinism, isolation
• Cache operations (6 tests): multi-layer get/set, invalidation
• @cached decorator (4 tests): caching, isolation, params
• Cache warming (4 tests): success, empty, error handling
• Statistics (2 tests): retrieval and reset
• Route endpoints (16 tests): all 8 endpoints with edge cases
• Auth requirements (2 tests): unauthorized access

Files Changed

File	Change
app/services/smart_cache.py	Multi-layer cache engine with L1/L2
app/routes/smart_cache.py	8 API endpoints
app/routes/__init__.py	Register cache blueprint
tests/conftest.py	Add fakeredis + smart_cache patching
tests/test_smart_cache.py	46 comprehensive tests
docs/smart-caching.md	Full documentation