Add backend API skeleton, schemas, and DB/test scaffolding

[Copilot]

This change establishes the FastAPI backend skeleton with required health/events/rec_centers endpoints, canonical schemas with example fixtures, and foundational DB/test tooling (SQLAlchemy/Alembic + pytest coverage). It also resolves the mobile CI dependency conflict in Flutter.

• API surface + fixtures
  • Add /health, /events, /rec_centers endpoints wired to mock fixture data.
  • Add fixtures under backend/fixtures/ and docs/examples/.
• Canonical schemas
  • Introduce Pydantic models (backend/schemas/*.py) and JSON schemas (docs/schemas/*.json) for events, rec centers, villages.
• DB + migrations scaffolding
  • Add SQLAlchemy base + database.py and Alembic config/migrations skeleton.
• Testing + coverage
  • Add pytest config and endpoint tests; enable coverage configuration.
• Mobile CI unblock
  • Downgrade flutter_timezone to a compatible version.

Example endpoint shape:

@app.get("/health")
async def health() -> dict[str, str]:
    return {"status": "ok"}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement FastAPI Backend Skeleton</issue_title>
<issue_description>Description: Create a FastAPI backend application exposing basic endpoints:

GET /health — returns {"status":"ok"}

GET /events (mock data)

GET /rec_centers (mock data)

Include automatic docs (/docs via Swagger UI).
Use SQLAlchemy + Alembic for DB migrations.
Add pytest + coverage config.

Acceptance Criteria:

Running uvicorn backend.app:app starts successfully.

GET /health returns status ok.

Unit tests for endpoints pass.

CI job executes pytest and reports success.
</issue_description>

<agent_instructions>Confirm all tasks from issue #2 are complete, all tests pass (if not, FIX ALL errors first), close issue, and proceed with issue #3 effectively and thoroughly. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement FastAPI Backend Skeleton #3

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