Refactor service types

[spotlightishere]

This implements some changes to allow for somewhat smoother refinements in future follow-up pull requests. Instead of having hardcoded integer checks everywhere, enum values are more heavily utilized throughout the codebase.

Note: Changes related to the API (e.g. its mixed use of network service names vs. their underlying service IDs) have not been thoroughly tested.

Future areas of improvement:

• Adopt SQLAlchemy's ORM model in many locations
• Attempt to split large while loop within backend.py once database-dependent code is modified
• ...?

[spotlightishere]

Going to go ahead and close this in favor of other approaches discussed within the development server. Appreciate the feedback given there!