remove distribution from hold, add vulnerability scanning in appview.

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).

pkg/appview/db/migrations/0011_fix_captain_successor_column.yaml

@@ -0,0 +1,25 @@
+description: Rebuild hold_captain_records to match schema.sql (provider→successor rename was missed when migration 0010 was recorded but not executed on a fresh DB)
+query: |
+  -- Recreate table to match schema.sql exactly
+  CREATE TABLE IF NOT EXISTS hold_captain_records_new (
+      hold_did TEXT PRIMARY KEY,
+      owner_did TEXT NOT NULL,
+      public BOOLEAN NOT NULL,
+      allow_all_crew BOOLEAN NOT NULL,
+      deployed_at TEXT,
+      region TEXT,
+      successor TEXT,
+      updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+  );
+
+  -- Copy data (only guaranteed-common columns; successor will be NULL)
+  INSERT OR IGNORE INTO hold_captain_records_new (hold_did, owner_did, public, allow_all_crew, deployed_at, region, updated_at)
+  SELECT hold_did, owner_did, public, allow_all_crew, deployed_at, region, updated_at
+  FROM hold_captain_records;
+
+  -- Swap tables
+  DROP TABLE hold_captain_records;
+  ALTER TABLE hold_captain_records_new RENAME TO hold_captain_records;
+
+  -- Recreate index
+  CREATE INDEX IF NOT EXISTS idx_hold_captain_updated ON hold_captain_records(updated_at);