feat: printable QR code endpoint for demo bags

[EricGrill]

Summary

• Adds GET /lot/:id/qr — returns a self-contained SVG with the provenance QR code, farm name, lot ID, and label
• SVG is print-ready: stick it on a coffee bag, scan it, see the full provenance chain
• 2 new tests (41 total, all passing)

Phase 2 #6. Closes #30.

Test plan

• GET /lot/:id/qr returns SVG with correct content-type
• SVG contains QR code, farm name, lot ID label
• Returns 404 for unknown lot
• All 41 tests pass

🤖 Generated with Claude Code