[I-019] [Improvement]: Normalize pagination responses across all list endpoints #375

Priority: Low
Phase: Phase 4 (API Polish)
Dependencies: #372

📝 User Story

As an API consumer,
I want to receive consistent pagination metadata from all list endpoints,
So that I can build generic pagination UI components.

🎯 Scope & Requirements

• Ensure all paginated endpoints (/api/collection, /api/entities, /api/email-templates, /api/workflows, /api/api-keys, /api/v1/entities) return the same pagination envelope
• Standard pagination response: { "data": [...], "pagination": { "total": N, "per_page": N, "current_page": N, "last_page": N } }
• Support ?page=N&per_page=N query parameters consistently
• Default per_page to a sensible value (e.g., 25)

✅ Acceptance Criteria

• Given any list endpoint, When called with pagination params, Then the response contains data and pagination keys.
• Given ?page=2&per_page=10, When sent to any list endpoint, Then the correct page of results is returned.

📋 Task Checklist

• Create a generic PaginatedResponse<T> wrapper
• Update all list handlers to use the wrapper
• Standardize query parameters
• Add integration tests for pagination