feat: smart caching strategy for dashboard & analytics

[mendarb]

Summary

• Add in-memory caching layer using Flask-Caching with SimpleCache backend
• Dashboard queries cached with 5-minute TTL, analytics with 15-minute TTL
• Cache key based on user_id + query parameters (MD5 hash)
• Cache invalidation triggered on expense, bill, and category CRUD operations
• Add X-Cache-Hit response header (true/false) for debugging
• Add flask-caching==2.3.0 dependency

Closes #127

Test plan

• First dashboard request returns X-Cache-Hit: false
• Second identical request returns X-Cache-Hit: true
• Creating/updating/deleting an expense invalidates dashboard cache
• Creating a bill invalidates dashboard cache
• Category CRUD (create, update, delete) each invalidate dashboard cache
• Different month query params produce separate cache entries
• Analytics (insights) endpoint uses 15-minute TTL

🤖 Generated with Claude Code