TASK-17: Add backend test suite with pytest

[z4ab]

Closes #20

What

Adds a comprehensive pytest test suite for the backend: unit tests for utility functions and integration tests for all API endpoints.

Changes

File	Description
backend/pytest.ini	Pytest config with asyncio_mode = auto
backend/tests/conftest.py	Session-scoped DB pool, per-test table truncation, sample seed data (6 projects across 5 cities), ASGI httpx client
backend/tests/test_queries.py	18 unit tests for haversine distance, bounding box, zoom-to-grid
backend/tests/test_api.py	17 integration tests covering health, list/filter/search projects, single project, submit, stats
backend/requirements.txt	Added pytest, pytest-asyncio, asgi-lifespan
db/setup_test_db.sh	Script to create and initialize the constructnear_test database
db/Makefile	Added test-db target

How to Run

# 1. Create test database (PostgreSQL must be running)
make -C db test-db

# 2. Run tests
pytest backend/tests -v

Or with custom DB URL:

DATABASE_URL_TEST=postgres://postgres:postgres@localhost:5432/constructnear_test pytest backend/tests -v

Test Coverage

Unit tests (18):

• Haversine: same point, NYC↔LA, NYC↔London, commutativity, antipodal, small distance
• Bounding box: NYC 5km, zero radius, large radius, ordering
• Zoom-to-grid: all tiers (8→18+), None, monotonic property

Integration tests (17):

• Health check
• List all, filter by type, filter by status
• Spatial search (within radius, exclude far-away)
• Pagination (limit/offset, disjoint pages)
• Clustering mode
• Empty results, invalid params (422)
• Get by ID, get nonexistent (404)
• Submit valid/minimal/invalid (422)
• Stats structure, counts, grouping

Verification

• ruff check . — passes
• ruff format --check . — passes