[PR 13] Usage Analytics + Audit

Implement skill download tracking and audit events for usage analytics.

## What to implement

### Audit Events
- `Operation::SkillDownload` event on pull/content retrieval
- `Operation::SkillSync` event on `/v1/` route hits (lazy mode)

### Download Counts
- Increment `download_count` on every `GET /v1/skill/...` hit
- Include in skill listing response

### UI
- Download badge on `SkillGridCard`
- Weekly usage chart on skill detail page

## Files to modify

- `crates/opsml_server/src/core/v1/route.rs` — increment download_count on /v1/ routes
- `crates/opsml_types/src/contracts/mod.rs` — add `Operation::SkillDownload`, `Operation::SkillSync`
- UI components — add download badge and usage chart

## Verification

```bash
cargo test -p opsml-server -- --nocapture --test-threads=1
# Pull skill → check audit log → verify count increments in listing response
```

## Depends on

- #361 (PR 12)

Check dependency #361 first, then read crates/opsml_server/src/core/v1/route.rs and crates/opsml_types/src/contracts/mod.rs. Run the provided cargo test command and locate the SkillGridCard and skill detail page UI components. Done means both audit operations are recorded, GET /v1/skill/... updates and exposes download_count, and the UI shows the badge and weekly usage chart.

Written by the indexing model from the issue text.