Remove the unused crypto path and write the blob-ID hash step once (closes #151)

Production encryption and decryption already run through blobgen; the crypto package (Encryptor, Decryptor, UpdateRecipients, the fx Module) and Vaultik.GetEncryptor/GetDecryptor had no production caller. Delete crypto and route verify --deep through the same blobgen reader restore uses, parsing the age key once.

The second blob-ID hash step is now one exported blobgen.DoubleSHA256; Writer.Sum256 (the double hash) becomes Writer.ContentID so it no longer collides with Reader.Sum256 (the single plaintext hash). Also delete the never-adopted internal/types newtypes and the uncalled CleanupIncompleteSnapshots and its now-dead deleteSnapshot caller, and correct ARCHITECTURE.md. No production behavior changes.

Model: opus-4-8

ARCHITECTURE.md

@@ -286,7 +286,6 @@ Key methods:
 - `CreateSnapshot(ctx, hostname, version, commit)` → Create snapshot record
 - `CompleteSnapshot(ctx, snapshotID)` → Mark snapshot complete
 - `ExportSnapshotMetadata(ctx, dbPath, snapshotID)` → Export to S3
-- `CleanupIncompleteSnapshots(ctx, hostname)` → Remove failed snapshots
 
 ### `internal/database`
 SQLite database for local index. Single-writer mode for thread safety.
@@ -307,7 +306,7 @@ Repository interfaces:
 ```
 CreateSnapshot(opts)
     │
-    ├─► CleanupIncompleteSnapshots()   // Critical: avoid dedup errors
+    ├─► PruneDatabase()   // Critical: avoid dedup errors
     │
     ├─► SnapshotManager.CreateSnapshot()   // Create DB record
     │