feat: serve lightweight HTML page on 503 errors

[elliotechne]

Summary

• Adds a minimal templates/503.html fallback page (no JS, no external resources, ~800 bytes inline CSS)
• Adds a @app.errorhandler(503) in error_handler.py that serves the HTML page to browser clients and JSON to API clients, using the Accept header to distinguish
• Addresses OOM-related 503s (from reduced Gunicorn workers / memory limits) so users see a readable page instead of a raw JSON error

Test plan

• Trigger a 503 (e.g. temporarily break DB connection or call the health endpoint with DB down) and confirm a browser GET returns the HTML fallback page with status 503
• Confirm an Accept: application/json request to a 503 endpoint still returns JSON {"error": "Service unavailable", ...}
• Verify the 503 page loads with no external network requests (pure inline styles)

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