feat: FastAPI app configuration

[jadegold55]

Summary

This branch is where the FastAPI application will be configured and built out.

• Adds the FastAPI app instance in src/polywatch/api/main.py — this is the central entry point for all API routes
• Adds src/polywatch/logger.py with a structlog-based logging setup (setup_logging) and a module-level log instance ready to use across the app

What's next

Further work on this branch will include:

• Route definitions and routers
• Dependency injection setup (DB sessions, auth, etc.)
• Middleware (CORS, request ID, OpenTelemetry tracing)
• Application lifespan events (startup/shutdown)