Placeholder: move Genre from fixed enum to DB-backed lookup table #310

Background

Placeholder for a previously-noted idea: move Genre from a fixed Quotinator.Core enum to a
DB-backed lookup table, so new genres don't require a recompile. Filed now as a tracked placeholder,
not scheduled for immediate implementation — ADR 018
(system-level content belongs in Quotinator.Data) names this as the second anticipated consumer of
its file-authored system content pattern, alongside System_Changelog.

What needs to be done

1. When this issue is actually planned, cross-check it against ADR 018's file-authored system content
   pattern and whichever generic importer abstraction the System_Changelog issue built — reuse it
   rather than designing a second one.
2. Design how existing Genre-valued data (the QuoteGenres join, any code referencing the Genre
   enum) migrates to a lookup-table-backed value — this is a real migration-safety concern (existing
   rows reference genre values by current enum member names) and needs its own careful design, not
   assumed to be a drop-in replacement.
3. Everything else deferred to this issue's own planning phase — this placeholder intentionally does
   not pre-design the solution.

Expected tests

Definition of done

• All expected tests listed above start red before implementation
• All requirements implemented
• All expected tests pass (green)
• No regression in related tests
• Findings summarised in a closing comment