[FEAT]: Digital Signature Support on Output PDFs

[krrishrastogi05]

📝 Description

FireForm's output PDFs are currently unsigned. For legal incident reports submitted to county sheriffs, EMS, and other agencies, a verifiable digital signature from the originating fire department is often a compliance requirement.

💡 Rationale

If FireForm is to be adopted as an official reporting tool by departments, the output must be legally defensible. A digitally signed PDF certifies the document's origin and integrity — critical for incident reports that may be used in investigations or court proceedings.

🛠️ Proposed Solution

• Integrate pyhanko for PDF signing
• Allow departments to upload a PKCS#12 certificate (.p12 / .pfx) via a configuration endpoint
• Add an optional sign: true flag to the fill request payload
• Apply a visible or invisible signature field to the final output PDF
• Add pyhanko to requirements.txt

✅ Acceptance Criteria

• Output PDFs can be optionally signed with a department-provided certificate
• Signature is verifiable in Adobe Acrobat and standard PDF viewers
• Unsigned behaviour is unchanged when sign: false (default)
• Works within the existing Docker container setup
• Documented in Swagger UI (/docs)