Correct the security claims in docs and comments, and record the accepted risks (closes #171)

Docs and comments only; no behaviour change. Corrects ten overclaims the
security review found: snapshot names are hashed but the hash uses no
secret, so a guessed hostname and name can be confirmed; a blob is named
by hex(SHA256(SHA256(uncompressed contents))), stated once in
docs/REPOSTRUCTURE.md and referenced elsewhere; double hashing does not
hide known content (blob packing does); age uses ChaCha20-Poly1305, not
XChaCha20; encryption is required, not optional; a snapshot is marked
complete before its metadata is uploaded; the export comment now matches
its only caller; deep verify detects corruption, not authorship; adding a
recipient does not reach existing data; restore examples target a
user-owned directory.

Adds an Accepted Risks subsection under Security Considerations with the
seven documented risks, cross-referenced from the README.

Model: opus-4-8

internal/cli/snapshot.go

@@ -192,7 +192,8 @@ func newSnapshotVerifyCommand() *cobra.Command {
 		Long: "Checks that every blob the snapshot's manifest lists is present\n" +
 			"in storage with the size the manifest records, and that the\n" +
 			"snapshot's encrypted database is present. It does not read blob\n" +
-			"contents; use --deep to download and cryptographically verify them.\n\n" +
+			"contents; use --deep to download, decrypt, and re-hash every blob\n" +
+			"to detect corruption -- integrity, not who wrote it.\n\n" +
 			"The snapshot may be named by its ID or, on a host with no local\n" +
 			"index, by the remote key that 'snapshot list' prints for a\n" +
 			"remote-only snapshot (an unambiguous leading part is enough).",

internal/database/database.go

@@ -3,8 +3,10 @@
 //
 // Blobs in Vaultik are the final storage units uploaded to S3. Each blob is a
 // large (up to 10GB) file containing many compressed and encrypted chunks from
-// multiple source files. Blobs are content-addressed, meaning their filename
+// multiple source files. Blobs are content-addressed: the filename in S3 is
-// is derived from their SHA256 hash after compression and encryption.
+// hex(SHA256(SHA256(uncompressed blob contents))), computed from the chunk data
+// before compression and encryption (not from the stored bytes). See
+// blobgen.DoubleSHA256 and docs/REPOSTRUCTURE.md.
 //
 // Schema is managed via numbered SQL migrations embedded in the schema/
 // directory. Migration 000.sql bootstraps the schema_migrations tracking