Count open reports that already say this

Answers “how many people have already reported this?” while the user is still filling in the form, so a second reporter is not silently deduped into a queue that then under-counts how often the problem is hit.

Not a window onto the queue: it returns one integer over a grouping tuple the caller already had to know in full, and never a report, a reporter or a comment. A partial subject answers zero rather than a validation error.

Session-only, and available wherever filing is.