Add the Database Reco Counts CSV report endpoint

[adamtheturtle]

The Cloud Targets Web API documents POST /imagetargets/databases/{database_id}/reports/recoCounts, which the mock does not route at all. It is the only documented Cloud Targets endpoint with no mock implementation.

The documented behaviour is:

• The request body is JSON with a month field in YYYY-mm form. Only the current and previous month can be requested.
• The response is {"result_code": "Success", "transaction_id": ..., "presigned_url": ...}.
• The presigned URL returns 404 until the report is ready, and then 200 with the CSV content.
• The CSV has one row per target with at least one reco in the month, with target_id and reco_count columns.

The mock does not count recos (see docs/source/differences-to-vws.rst), so a mock report will necessarily be empty or synthetic.

Acceptance criteria

• The VWS mock routes POST /imagetargets/databases/{database_id}/reports/recoCounts on both the in-process and Flask backends.
• A presigned-URL-like download route serves the generated CSV, including the documented 404-until-ready behaviour.
• Verified fake tests cover a valid request, an out-of-range month, and a malformed month.
• Any remaining differences, in particular the empty reco counts, are documented in docs/source/differences-to-vws.rst.