Add database indexes and caching for parameter-values endpoint

[anth-volk]

Fixes #49

Summary

• Add database indexes to parameter_values table for query optimization
• Add response caching (1 hour TTL) to the /parameter-values/ endpoint

Changes

Database Migration (supabase/migrations/20251229000000_add_parameter_values_indexes.sql)

Creates 4 indexes:

• Composite index on (parameter_id, policy_id) for combined filtering
• Single index on policy_id for policy-only queries
• Partial index on parameter_id WHERE policy_id IS NULL for baseline value lookups
• Index on start_date for time-range queries

Caching (src/policyengine_api/api/parameter_values.py)

Added @cache(expire=3600) decorator to list_parameter_values, matching the pattern used in variables.py.

Test plan

• All 9 tests in test_parameters.py pass
• Linter passes
• After deploy, verify indexes are used with EXPLAIN ANALYZE
• Benchmark endpoint response time

🤖 Generated with Claude Code