[Pipeline C] Form generation: generate endpoint, fill task, retrieval endpoints (#552)

[abhishek-8081]

Part 2 of #552 (part 1 added the Form model + migration). This builds the full form
generation flow — generate, fill, and retrieve.

What it does

A user picks the ready templates on the readiness screen and submits them. This PR takes that
request, generates a filled PDF (and a JSON version) for each template by reading values
straight from the incident's contract, and lets the user poll for progress and download the
results. No LLM runs at this stage — filling is pure lookup and draw, so a batch finishes in
seconds.

Endpoints

• POST /forms/generate — validates the incident, checks each requested template's
  readiness, and splits them into queued (ready) and skipped (not ready, with a reason).
  Creates a batch + one Form row per queued template, dispatches the fill task, and returns a
  BatchGenerateResponse. template_ids is required here; auto-selecting every ready template
  is Batch form generation - API Contracts #554.
• GET /forms/batch/{batch_id} — batch status (counts + per-form status), derived on the
  fly from the Form rows (no Batch table, per the agreed scope).
• GET /forms/{form_id} — the form record.
• GET /forms/{form_id}/pdf — the generated PDF.
• GET /forms/{form_id}/json — the agency-field-mapped JSON.

The fill task

For each form in the batch, independently:

• Resolve every template field's value from the incident contract (reuses resolve() from
  extraction_readiness).
• Draw the placed fields onto a ReportLab overlay at their layout coordinates, then merge that
  overlay onto the template PDF with pypdf.
• Save the PDF under data/forms/generated/{form_id}.pdf, build the mapped JSON, and compute
  the field-mapping summary (reuses gaps_for()).

Each form is wrapped in its own try/except — if one form fails, it's marked failed and the
batch carries on, so a single bad form never sinks the whole job.

Reuse and structure

Reuses resolve() and gaps_for() from the readiness engine, and mirrors the extraction
service/worker/task split rather than the legacy fill path. Adds reportlab + Pillow.

Verification

• 485 tests pass (454 baseline + 31 new). ruff clean.
• Verified the coordinate drawing visually: values land on their boxes with the correct
  left/center/right alignment and no vertical flip (bottom-left origin, matching ReportLab).

Three API-surface calls — implemented this way, open to your thoughts

The contract doesn't fully spell these out, so I made a call on each. They're implemented and
working as described below — happy to change any if you'd prefer a different approach:

1. GET /forms/{form_id}/pdf: a failed form returns 500 (PDF_GENERATION_FAILED); one not ready
   yet returns 202 with a retry hint.
2. output_format is accepted but not honored yet — the fill always writes both PDF and JSON.
   Honoring pdf-only / json-only felt like Batch form generation - API Contracts #554 scope.
3. BatchStatus reads "completed" even when some forms failed (the per-form statuses still show
   exactly which ones), matching the per-form isolation we agreed on.