Enhance topic payload contracts with topic type and endpoint-first spec

[mottych]

Summary

• extend runtime OpenAPI topic contract index to include both single-shot and conversation topics
• include topicType discriminator per topic for unified client routing
• add conversation payload schema refs and coaching start examples to the OpenAPI contract enrichment
• add tests for new contract behavior, including goals conversation topic
• simplify unified AI integration spec to endpoint-first guidance and direct consumers to runtime OpenAPI/AsyncAPI as source of truth

Why

Frontend clients need a single machine-readable topic payload contract that supports different payload/response schemas by topic type without duplicating static documentation.

Validation

• uv run pytest tests/unit/api/test_openapi_topic_contracts.py tests/unit/core/test_endpoint_registry.py tests/unit/models/test_coaching_results.py
• uv run ruff check src/api/openapi_topic_contracts.py src/models/coaching_results.py src/core/topic_registry.py src/core/topic_seed_data.py tests/unit/api/test_openapi_topic_contracts.py tests/unit/core/test_endpoint_registry.py tests/unit/models/test_coaching_results.py
• uv run mypy src/api/openapi_topic_contracts.py tests/unit/api/test_openapi_topic_contracts.py --explicit-package-bases

Fixes #321