Add evidence-backed Interview Packet V1

[mergemaven11]

What changed

• adds a Pro-gated /packets/interview endpoint built from user-selected accomplishments
• supports 1–8 selected stories from the active report period
• adds optional target role and target organization framing
• builds interview stories from documented contribution, result, skills, proof status, and evidence counts
• surfaces preparation questions when contribution/result/skills are missing instead of inventing STAR details
• keeps evidence references private by default and only includes them when the user explicitly enables export
• adds a dedicated server-generated Interview Packet PDF at /packets/interview.pdf
• adds Interview Packet preview pages for selected stories, skills/outcomes, target framing, and interview prep questions
• adds Interview Packet as the third Pro packet type alongside Performance Review and Promotion
• adds dedicated interview_packet entitlements for Pro, Team, and Enterprise
• adds backend coverage using a non-technical education scenario
• refreshes README positioning so BragStack is clearly a career evidence system for all professions and reflects the packet/PDF features already shipped

Product principles

• user chooses the stories
• evidence remains private by default
• no invented STAR details, metrics, claims, or evidence
• same career-neutral evidence model across professions
• interview preparation is a user-support artifact, not an automated hiring decision

Validation target

• Backend CI
• Frontend ESLint
• Frontend production build
• Free entitlement rejection
• selected-story filtering
• evidence-reference privacy default
• explicit evidence-reference export
• real PDF generation and interview-specific filename

Closes #27.