feat(snapshots): Add download command for baseline snapshots (#3310)

## Summary

- Adds `sentry-cli snapshots download` command for downloading baseline
snapshot images from Sentry's preprod system to a local directory
- Supports `--app-id` (resolves latest baseline snapshot) or
`--snapshot-id` (direct artifact ID), with optional `--branch` filter
- Downloads a ZIP from the server and extracts images to `--output`
directory (default `./snapshots-base/`)
- Adds two API methods: `get_latest_base_snapshot` and
`download_snapshot_zip`

### Usage

```bash
# Download latest baselines for an app
sentry-cli snapshots download --app-id sentry-frontend

# Download from a specific branch
sentry-cli snapshots download --app-id sentry-frontend --branch main

# Download a specific snapshot by ID
sentry-cli snapshots download --snapshot-id 259299

# Full local visual regression workflow
sentry-cli snapshots download --app-id sentry-frontend
pnpm run snapshots
sentry-cli snapshots diff ./snapshots-base .artifacts/snapshots --fail-on-diff
```

## Test plan

- [x] `cargo clippy -- -Dwarnings` passes clean
- [x] `cargo fmt --check` passes
- [x] `cargo test snapshots` — all 4 integration tests pass (including
new download help test)
- [ ] Manual test: run `sentry-cli snapshots download --app-id
sentry-frontend` against real Sentry org

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: NicoHinderling

.github/CODEOWNERS

@@ -19,6 +19,8 @@
 /src/commands/snapshots @getsentry/emerge-tools @getsentry/owners-sentry-cli
 /src/utils/odiff @getsentry/emerge-tools @getsentry/owners-sentry-cli
 /tests/integration/build @getsentry/emerge-tools @getsentry/owners-sentry-cli
+/tests/integration/snapshots.rs @getsentry/emerge-tools @getsentry/owners-sentry-cli
+/tests/integration/_cases/snapshots @getsentry/emerge-tools @getsentry/owners-sentry-cli
 
 # Files without codeowner protection
 /CHANGELOG.md

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Features
 
 - (snapshots) Add `snapshots diff` command for locally comparing directories of PNG snapshot images using odiff ([#3306](https://github.com/getsentry/sentry-cli/pull/3306))
+- (snapshots) Add `snapshots download` command for downloading baseline snapshot images from Sentry ([#3310](https://github.com/getsentry/sentry-cli/pull/3310))
 
 ### Fixes