Store and expose the database load timestamp

[thekaveman]

It would be helpful to know the timestamp of when the database was last (re)loaded.

Idea:

• During db-init, create a new table to store a single record with the current timestamp when db-init is called
• Create an endpoint like /metadata that returns a JSON blob like:

{
  "app": {
    "version": "version string"
  },
  "db": {
    "timestamp": "stored timestamp",
    "users": 123, //count of users in DB
    "eligibility": ["list", "of", "types"]
   }
}

Acceptance Criteria

• We have an easy way to tell when the database was last refreshed, and some basic stats about the current state of the app/database

Additional context

Related to debugging cal-itp/benefits#2598

[thekaveman]

PR #512 is up and the code is ready and working.

But we're currently blocked by #511 as this PR will need to build/deploy the infrastructure pipeline.