backend-servicelisted

# Add a backend service Implement domain logic as a focused, testable service that keeps transport (HTTP) and persistence (DB) concerns at the edges, following the repo's layering. ## Steps 1. **Read the lore first.** Call `search_lore` (Memory MCP) for the repo's layering (controller → service → repository), error model, transaction handling, and dependency-injection conventions. Honour any ADRs. 2. **Find a sibling service** and copy its shape — constructor/factory, injected collaborators, return/error types, and where it lives. 3. **Keep the service transport-agnostic.** It receives validated inputs and returns domain results or typed errors — no HTTP request/response objects, no SQL strings. 4. **Depend on abstractions.** Take repositories/clients via interfaces so the logic is unit-testable with fakes; favour immutable inputs and outputs. 5. **Handle failure explicitly.** Map domain failures to typed errors/results; never swallow exceptions or return ambiguous nulls. 6. **Test the logic in isolation.** Unit-test happy path, branches, and error cases with collaborators faked; add an integration test if it crosses real boundaries. 7. **Verify + evidence.** Run tests and lint, then record `test_output` via the `record-evidence` skill and submit for review. ## Rules - One responsibility per service; keep methods small and the file cohesive. - No transport or storage details leaking into the service layer. - Immutable inputs/outputs; explicit erro