[APIView] Super Epic: Version-Centric Data Model

## Overview

Master tracking issue for the APIView Version-Centric Data Model — a redesign of how APIView stores and operates on package versions, comments, approvals, and PR lifecycles.

This epic addresses six structural problems in the current data model: comment bleed across revisions, version ambiguity, opaque approval copying, duplicate blob storage, comment orphaning, and daily alpha/dev version churn.

## Area Epics

- [ ] #15010 — Foundation: APIVersion Entity & Data Migration (WI-1, WI-2, WI-3)
- [ ] #15011 — Retention: Unified Retention System (WI-4, WI-5, WI-6)
- [ ] #15012 — Comments: Version-Scoped Comments (WI-7, WI-8, WI-9)
- [ ] #15013 — PR Lifecycle: Version-Aware PRs (WI-10, WI-11, WI-12, WI-13)
- [ ] #15014 — Approvals: Version-Level Approvals (WI-14, WI-15, WI-16, WI-17)

## High-Level Dependency Map

```
Foundation ──┬── Retention
├── Comments → PR Lifecycle (partially)
├── PR Lifecycle
└── Approvals
```

Every area depends on Foundation (WI-1 through WI-3). Retention and Approvals each have one work item (WI-4, WI-14) that can start in parallel with Foundation. PR Lifecycle's promotion-on-merge (WI-12) depends on Comments (WI-8).

## Stable-State Gates

| Gate | Work Items | Constraint |
|------|-----------|------------|
| Comment query activation | WI-7 verified → flip WI-8 | Comments without `APIVersionId` would be invisible |
| Approval activation | WI-15 + WI-17 flip together | Backend and SPA must agree on version-level approval |

## Issues Resolved

https://github.com/Azure/azure-sdk-tools/issues/14187
https://github.com/Azure/azure-sdk-tools/issues/5186
https://github.com/Azure/azure-sdk-tools/issues/10105
https://github.com/Azure/azure-sdk-tools/issues/8634
https://github.com/Azure/azure-sdk-tools/issues/9595
https://github.com/Azure/azure-sdk-tools/issues/12121

# Original Issue

https://github.com/Azure/azure-sdk-tools/issues/14758

## Approved Proposal

[versionProposal.md](https://github.com/user-attachments/files/26581422/versionProposal.md)