Rework form generation around incidents and templates

Why

Form generation was still built around a single extraction and a closed list of form types, from before templates were user-registered and before extraction started producing a stored incident. Two separate endpoints for one form versus all forms did not make sense once generation is just a lookup, not an LLM call.

What changed

• generate/all and generate/{form_type} are now one POST /forms/generate, taking an incident_id and a list of template_ids. Leaving template_ids out generates everything the readiness matrix reports as ready.
• Generation reads from incident_id, since the contract document now lives on the incident row rather than the extraction.
• Generation stays a pure lookup and draw, no LLM, so a whole batch finishes in seconds regardless of how many templates are requested.
• Templates that are not ready are skipped with a reason instead of failing the whole request, unless force_partial is set.
• Single-form and batch responses now share the same FormRecord shape.
• Added GET /forms/batch/{batch_id}/download to pull a batch as one archive.

Reference

Discussion: #609.
Related Issue: #619