[APIView] Epic: Version Entity & Data Migration

## Overview

Foundation work for the APIView Version-Centric Data Model. Introduces the `APIVersion` entity, wires it into all upload paths, and backfills historical data.

## Work Items

- [ ] #14992 — WI-1: APIVersion Entity & Repository Layer
- [ ] #14993 — WI-2: Wire Upload Paths
- [ ] #14994 — WI-3: Backfill Revisions & Samples

## Dependency Chain

```
WI-1 → WI-2 → WI-3
```

WI-1 has no dependencies and is the starting point for the entire version-centric model. WI-3 is a prerequisite for most downstream work (Retention, Comments, PR Lifecycle, Approvals).