backend/docs: OpenAPI descriptions for ingest and health routes

[CodewithJha]

Problem

GOOD_FIRST_ISSUES #38: public routes lack rich OpenAPI docstrings. Ingest (/api/ingest/v1/*) and health are the surfaces external integrators hit first. FastAPI will expose schemas, but summaries/descriptions are thin.

Repository evidence

• apps/api/src/mutiny_api/ingest.py / app.py route handlers
• schemas.py / ingest_schemas.py
• Related open issue backend/docs: document /api/health response fields #8 (health docs page) — this issue is OpenAPI/operation descriptions in code

Proposed direction

Add concise summary/description on 2–4 critical routes (health, ingest open, ingest events, ingest artifact) stating observe-only / redaction required / auth when configured.

Acceptance criteria

• OpenAPI JSON (/openapi.json) shows improved descriptions for chosen routes
• Text states Hosted does not execute customer adapters
• No behavior change

Difficulty

S · good first issue