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/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
-// is derived from their SHA256 hash after compression and encryption.
+// multiple source files. Blobs are content-addressed: the filename in S3 is
+// 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