[NO-ISSUE] Implement database consistency checks at startup

[tchoutri]

This PR implements consistency checks at startup. For now they are quite rudimentary:

• Checks if the repo update job is queued, if not it's being enqueued
• Checks if all the expected package indexes are already provisioned in the database, if not it stops the server. You need to run make db-provision.

Example output:

SELECT "package_indexes"."repository" FROM "package_indexes"
2024-04-02 18:06:27 ATTENTION flora-server: Database validation failed: Expected package indexes: horizon.
make: *** [Makefile:5: start] Error 1