feat: add RequestShaper database schema

[TheArchitectit]

Part 3: Database Schema (53 lines)

Add database schema for RequestShaper service.

Size: 53 lines ✅

[TheArchitectit]

Code Overview

This PR adds database schema for RequestShaper persistence.

Key Changes:

1. packages/backend/drizzle/schema/sqlite/provider-rate-limits.ts - SQLite schema
2. packages/backend/drizzle/schema/postgres/provider-rate-limits.ts - PostgreSQL schema
3. packages/backend/drizzle/migrations/0016_hard_vulcan.sql - Migration file
4. packages/backend/drizzle/schema/index.ts - Export updates

Schema:

• provider_rate_limits table
• Fields: provider, model, alias, current_budget, last_refill_at
• Indexes for lookup performance

Migration:

• Run: npm run db:migrate
• Backward compatible (new table, no breaking changes)

Testing:

• Migration applies cleanly to both SQLite and PostgreSQL