✨ Add built-in tenant-aware APQ caching support

[evoludigit]

Summary

Implements built-in tenant-aware caching for Automatic Persisted Queries (APQ), enabling secure multi-tenant applications with automatic response isolation.

Key Changes

• Built-in tenant isolation in base backends - Both MemoryAPQBackend and PostgreSQLAPQBackend now automatically isolate cached responses by tenant when context is provided
• Context propagation through APQ stack - Router passes context to backend methods for tenant-aware operations
• Zero configuration required - Works out of the box, no custom implementations needed

Implementation Details

MemoryAPQBackend

• Generates tenant-specific cache keys: {tenant_id}:{hash}
• Maintains separate cache spaces per tenant
• Global cache still available for non-tenant requests

PostgreSQLAPQBackend

• Added tenant_id column to responses table
• Composite primary key (hash, COALESCE(tenant_id, ''))
• Indexed tenant_id for performance

Security

• Prevents cross-tenant data leakage
• Each tenant only sees their own cached responses
• Validated through comprehensive test suite

Testing

• ✅ All 3287 tests passing
• ✅ Added tenant isolation tests
• ✅ Context propagation tests
• ✅ Backward compatibility tests

Usage

# Just pass context with tenant_id - isolation is automatic!
context = {"user": {"metadata": {"tenant_id": "acme-corp"}}}
response = backend.get_cached_response(hash, context=context)

Documentation

• Added comprehensive guide: docs/apq_tenant_context_guide.md
• Updated example: examples/apq_multi_tenant.py
• Clean, focused implementation without transitional artifacts

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