fix: add Pydantic response models to 25 untyped API endpoints

[Sealjay]

Context

Backend schema audit found 39% of endpoints (25/64) lack proper response_model annotations, returning dict[str, Any]. This produces unknown types in the generated TypeScript SDK.

Additionally, 11 endpoints accept raw dict[str, Any] request bodies instead of Pydantic models.

High-priority endpoints needing response models

• GET /character/{id} — should use existing CharacterSheet
• POST /character/{id}/level-up — should use existing LevelUpResponse
• POST /dice/roll — create DiceRollResponse
• GET /campaign/templates — use CampaignListResponse
• All session endpoints (10 routes) — create typed models
• All combat init/turn endpoints (4 routes) — create typed models

High-priority endpoints needing request models

• POST /combat/initialize — create CombatInitializeRequest
• POST /combat/{id}/turn — create CombatTurnRequest
• POST /campaign/generate-world — create GenerateWorldRequest
• POST /session/{id}/action — create PlayerActionRequest

Tasks

• Add response_model to all 25 endpoints
• Create Pydantic request models for 11 raw-dict endpoints
• Regenerate openapi.json and schema.d.ts
• Verify bunx tsc --noEmit passes