[Go] Implement N+1 Query Solution with JOINs

[davideme]

Task

Implement the ?include= query parameter for eager loading in Go/sqlc.

Endpoints

• GET /buildings with ?include=rooms or ?include=rooms.lamps
• GET /buildings/{id} with ?include=rooms.lamps
• GET /buildings/{id}/rooms with ?include=lamps
• GET /rooms/{id} with ?include=lamps,building

Functional Requirement

Must minimize database queries regardless of the number of entities returned.

Implementation Hints

• Write SQL queries with JOINs in sqlc
• Create separate query variants for different include combinations
• Or use dynamic query building with proper JOIN clauses
• Map flattened results back to nested structs

Acceptance Criteria

• ?include=rooms returns buildings with nested rooms
• ?include=rooms.lamps returns buildings with rooms and lamps
• Query count does not increase with number of buildings (verify with SQL logging)

Parent Issue

Relates to #289