did:plc Identity Support (pkg/hold/pds/did.go, pkg/hold/config.go, pkg/hold/server.go)

The big feature — holds can now use did:plc identities instead of only did:web. This adds:
- LoadOrCreateDID() — resolves hold DID by priority: config DID > did.txt on disk > create new
- CreatePLCIdentity() — builds a genesis operation, signs with rotation key, submits to PLC directory
- EnsurePLCCurrent() — on boot, compares local signing key + URL against PLC directory and auto-updates if they've drifted (requires rotation key)
- New config fields: did_method (web/plc), did, plc_directory_url, rotation_key_path
- GenerateDIDDocument() now uses the stored DID instead of always deriving did:web from URL
- NewHoldServer wired up to call LoadOrCreateDID instead of GenerateDIDFromURL

CAR Export/Import (pkg/hold/pds/export.go, pkg/hold/pds/import.go, cmd/hold/repo.go)

New CLI subcommands for repo backup/restore:
- atcr-hold repo export — streams the hold's repo as a CAR file to stdout
- atcr-hold repo import <file>... — reads CAR files, upserts all records in a single atomic commit. Uses a bulkImportRecords method that opens a delta session, checks each record for
create vs update, commits once, and fires repo events.
- openHoldPDS() helper to spin up a HoldPDS from config for offline CLI operations

Admin UI Fixes (pkg/hold/admin/)

- Logout changed from GET to POST — nav template now uses a <form method=POST> instead of an <a> link (prevents CSRF on logout)
- Removed return_to parameter from login flow — simplified redirect logic, auth middleware now redirects to /admin/auth/login without query params

Config/Deploy

- config-hold.example.yaml and deploy/upcloud/configs/hold.yaml.tmpl updated with the four new did:plc config fields
- go.mod / go.sum — added github.com/did-method-plc/go-didplc dependency

1. Removing distribution/distribution from the Hold Service (biggest change)
The hold service previously used distribution's StorageDriver interface for all blob operations. This replaces it with direct AWS SDK v2 calls through ATCR's own pkg/s3.S3Service:
- New S3Service methods: Stat(), PutBytes(), Move(), Delete(), WalkBlobs(), ListPrefix() added to pkg/s3/types.go
- Pull zone fix: Presigned URLs are now generated against the real S3 endpoint, then the host is swapped to the CDN URL post-signing (previously the CDN URL was set as the endpoint, which
broke SigV4 signatures)
- All hold subsystems migrated: GC, OCI uploads, XRPC handlers, profile uploads, scan broadcaster, manifest posts — all now use *s3.S3Service instead of storagedriver.StorageDriver
- Config simplified: Removed configuration.Storage type and buildStorageConfigFromFields(); replaced with a simple S3Params() method
- Mock expanded: MockS3Client gains an in-memory object store + 5 new methods, replacing duplicate mockStorageDriver implementations in tests (~160 lines deleted from each test file)
2. Vulnerability Scan UI in AppView (new feature)
Displays scan results from the hold's PDS on the repository page:
- New lexicon: io/atcr/hold/scan.json with vulnReportBlob field for storing full Grype reports
- Two new HTMX endpoints: /api/scan-result (badge) and /api/vuln-details (modal with CVE table)
- New templates: vuln-badge.html (severity count chips) and vuln-details.html (full CVE table with NVD/GHSA links)
- Repository page: Lazy-loads scan badges per manifest via HTMX
- Tests: ~590 lines of test coverage for both handlers
3. S3 Diagnostic Tool
New cmd/s3-test/main.go (418 lines) — tests S3 connectivity with both SDK v1 and v2, including presigned URL generation, pull zone host swapping, and verbose signing debug output.
4. Deployment Tooling
- New syncServiceUnit() for comparing/updating systemd units on servers
- Update command now syncs config keys (adds missing keys from template) and service units with daemon-reload
5. DB Migration
0011_fix_captain_successor_column.yaml — rebuilds hold_captain_records to add the successor column that was missed in a previous migration.
6. Documentation
- APPVIEW-UI-FUTURE.md rewritten as a status-tracked feature inventory
- DISTRIBUTION.md renamed to CREDENTIAL_HELPER.md
- New REMOVING_DISTRIBUTION.md — 480-line analysis of fully removing distribution from the appview side
7. go.mod
aws-sdk-go v1 moved from indirect to direct (needed by cmd/s3-test).