Refactor API into modular routers

[Presmanes3]

Problem

The file src/climatevision/api/main.py has ~980 lines with all routes defined inline in create_app(). This makes maintenance, testing, and team collaboration difficult.

Proposed solution

Split routes into separate modules using FastAPI's APIRouter:

src/climatevision/api/
├── __init__.py
├── main.py              # Clean create_app() with include_routers()
├── routes/
│   ├── __init__.py
│   ├── health.py         # GET /api/health
│   ├── analysis.py      # GET /api/analysis-types
│   ├── runs.py          # CRUD /api/runs
│   ├── predict.py      # POST /api/predict, /api/predict/upload
│   ├── organizations.py # CRUD /api/organizations
│   ├── subscriptions.py # /api/organizations/{id}/subscriptions
│   └── alerts.py         # /api/organizations/{id}/alerts

Tasks

• [ ] Create api/routes/ with __init__.py
• [ ] Move each route group to its own module with APIRouter
• [ ] Clean main.py to only use include_routers()
• [ ] Move specific schemas or keep in schemas.py
• [ ] Verify all endpoints still work
• [ ] Update tests to reflect new structure