Problem

There is no dedicated endpoint for browsing the scheme catalog — clients currently have
to use the semantic search endpoint even when they just want to list or filter schemes by
structured attributes (agency, planning area, scheme type).

Link to Issue:

#268
dependent on #280

Changes

• Add catalog Cloud Function (backend/functions/schemes/catalog.py) — a new GET endpoint
  that returns paginated scheme results, with optional filtering by agency, area, or
  scheme_type
• Add catalog_pagination.py utility (backend/functions/utils/catalog_pagination.py) with
  signed cursor-based pagination using HMAC-SHA256
• Register catalog endpoint in main.py and add it to the keepalive warmup scheduler
  (endpoints.py)
• Add 3 Firestore composite indexes for the new filter+sort query shapes
  (firestore.indexes.json)
• Add integration tests (backend/tests/integration/test_catalog.py) and a
  test-endpoint.http file for manual testing

Why

The search endpoint is optimised for semantic/fuzzy queries and unsuitable for structured
browsing. A dedicated catalog endpoint allows consumers to page through schemes by
category without search overhead. Cursor-based pagination with HMAC signing prevents cursor tampering. Supporting only one filter at a time keeps query complexity manageable
while still covering the main use cases.

Testing

• Tested locally via Firebase emulator using test-endpoint.http — verified paginated
  responses, filter combinations, cursor continuity, and error cases (invalid params,
  unknown params, multiple filters)
• Integration tests pass (pytest backend/tests/integration/test_catalog.py)