Add evidence-backed Promotion Packet V1

[mergemaven11]

What changed

• adds a Pro-gated /packets/promotion endpoint that reuses the same evidence engine as the Performance Review Packet
• adds optional target role and target level/progression fields without baking a profession into the schema
• builds a deterministic promotion case from documented accomplishments, Impact Receipts, measurable outcomes, skills, evidence, shared credit, and confirmations
• adds career-neutral sections for demonstrated impact, scope & ownership, growth & capabilities, verified recognition, and concrete ways to strengthen the evidence case
• explicitly avoids automated promotion-readiness scores or employment decisions
• adds a dedicated server-generated Promotion Packet PDF at /packets/promotion.pdf
• adds a Promotion Packet physical preview with promotion-specific cover, scorecard framing, case pages, receipt appendix, evidence index, and conversation summary
• lets the existing Pro packet builder switch between Performance Review Packet and Promotion Packet
• adds direct Promotion PDF download using the authenticated blob-download flow
• adds backend coverage using a non-technical Operations Supervisor → Senior Operations Manager scenario

Product principle

Promotion is a human/organizational decision. BragStack makes the evidence case dramatically easier to assemble and inspect without pretending it can decide readiness, level, compensation, or an employment outcome.

Validation

• Backend CI ✅
• Promotion API entitlement test ✅
• Non-tech promotion-case test ✅
• Dedicated Promotion PDF generation test ✅
• Frontend ESLint ✅
• Frontend production build ✅

Stack

This PR intentionally targets feature/performance-review-packet-v1 and should land after #19.

Closes #26.