docs(epic-3): add tech spec and story drafts for PDF Export

Epic 3: PDF Export - Planning Phase Complete

Tech Spec:
- Create tech-spec-epic-3.md with 10 acceptance criteria
- Define API contracts, workflows, and NFRs
- Document dependencies (puppeteer, @sparticuz/chromium)
- Add Vercel deployment configuration
- Include traceability mapping and test strategy

Story Drafts:
- 3-1: Create PDF Generation API Route (8 ACs, Puppeteer integration)
- 3-2: Create PDF Download Composable (8 ACs, download logic)
- 3-3: Connect Download Button to PDF Generation (8 ACs, integration)

Sprint Status Updates:
- epic-3: backlog → contexted
- All 3 stories: backlog → drafted

Files Created:
- docs/sprint-artifacts/tech-spec-epic-3.md
- docs/sprint-artifacts/3-1-create-pdf-generation-api-route.md
- docs/sprint-artifacts/3-2-create-pdf-download-composable.md
- docs/sprint-artifacts/3-3-connect-download-button-to-pdf-generation.md

Files Modified:
- docs/sprint-artifacts/sprint-status.yaml

Ready for: Story context generation or implementation

docs/sprint-artifacts/sprint-status.yaml

@@ -60,8 +60,8 @@ development_status:
   # Goal: Enable high-quality PDF generation matching web preview
   # FRs: FR10-14
   # ═══════════════════════════════════════════════════════════════
-  epic-3: backlog
-  3-1-create-pdf-generation-api-route: backlog
-  3-2-create-pdf-download-composable: backlog
-  3-3-connect-download-button-to-pdf-generation: backlog
+  epic-3: contexted
+  3-1-create-pdf-generation-api-route: drafted
+  3-2-create-pdf-download-composable: drafted
+  3-3-connect-download-button-to-pdf-generation: drafted
   epic-3-retrospective: optional