feat: ACP/UCP protocol schema export

[antoniomtz]

Summary

• Add a Protocols tab to the enrichment UI with ACP/UCP sub-tabs for exporting enriched product data as commerce protocol-compliant JSON
• New POST /protocols/generate backend endpoint that calls the LLM once to extract structured fields (brand, material, age_group, product_details, etc.) and builds both ACP and UCP schemas
• UCP follows the Google Merchant Center Product Data Specification with structured_title/structured_description using digital_source_type: "trained_algorithmic_media"
• Schema generation fires in the background after FAQ generation completes, ensuring FAQs are included in both schemas

Changes

Backend (3 files):

• src/backend/main.py — Protocol schema builders with DRY helpers, /protocols/generate endpoint
• src/backend/vlm.py — _call_nemotron_extract_schema_fields() for LLM-based attribute extraction

Frontend (4 files):

• src/ui/components/ProtocolsTabContent.tsx — New component with segmented control, syntax-highlighted JSON, copy button
• src/ui/components/FieldsCard.tsx — Third tab wired in
• src/ui/app/page.tsx — Background protocol generation chained after FAQs
• src/ui/lib/api.ts — generateProtocolSchemas() API client

Tests (1 file):

• tests/test_protocol_schemas.py — 31 unit tests covering enriched field population, LLM field merging, deterministic defaults, nested types, edge cases

Docs (4 files):

• docs/API.md — Section 6 with full endpoint spec, response schema, curl example
• docs/PRD.md — FR-13 functional requirement + implementation TODO
• AGENTS.md — Status line + API endpoint entry
• README.md — Key Features bullet + API Endpoints list

Test plan

• Upload a product image and run analysis
• Verify Details and FAQs tabs still work (no regression)
• Switch to Protocols tab — should show spinner then populated schemas
• Toggle ACP/UCP — verify different structures with syntax highlighting
• Verify ACP has brand, product_details, highlights, FAQs populated by LLM
• Verify UCP has structured_title/description, brand, google_product_category, product_detail, FAQs
• Click Copy — verify plain JSON copied to clipboard
• Run pytest tests/ — all 239 tests pass
• Run pnpm build — clean build, no errors

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