feat: smart caching with intelligent invalidation (#127)

[alexchenai]

Closes #127

Implements intelligent caching with automatic invalidation for FinMind.

Features:

• Per-entity TTL policies (dashboard 5m, expenses 2m, categories 1h, bills 10m, insights 15m)
• Invalidation dependency graph: changing expenses auto-invalidates dashboard + analytics + insights
• Decorator-based route caching via @cached_route(entity)
• Cache warming support for login flow
• Cache statistics endpoint (/cache/stats) with hit rates per entity
• User cache clear endpoint (/cache/clear)
• Integrated with expenses route for automatic invalidation on create/update/delete
• 15 test cases covering key generation, TTL policies, invalidation deps, cache clear, and stats

Tech: Python, Flask, Redis, pytest

Files changed:

• packages/backend/app/services/smart_cache.py (new - core caching logic)
• packages/backend/app/routes/cache_admin.py (new - admin endpoints)
• packages/backend/app/routes/init.py (updated - register new blueprint)
• packages/backend/app/routes/expenses.py (updated - add smart invalidation)
• packages/backend/tests/test_smart_cache.py (new - test suite)