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/snapshot/snapshot.go

@@ -24,7 +24,7 @@ package snapshot
 // 7. Close the temporary database
 // 8. VACUUM the database to remove deleted data and compact (security critical)
 // 9. Compress the binary database with zstd
-// 10. Encrypt the compressed database with age (if encryption is enabled)
+// 10. Encrypt the compressed database with age (always; recipients are required)
 // 11. Upload to S3 as: metadata/{snapshot-id}/db.zst.age
 // 12. Reopen the main database
 //
@@ -238,14 +238,12 @@ func (sm *SnapshotManager) CompleteSnapshot(
 // 3. Cleans the copy to contain only current snapshot data
 // 4. Dumps the cleaned database to SQL
 // 5. Compresses the SQL dump with zstd
-// 6. Encrypts the compressed data (if encryption is enabled)
+// 6. Encrypts the compressed data with age (always; recipients are required)
 // 7. Uploads to S3 at: snapshots/{snapshot-id}.sql.zst[.age]
 //
-// The caller is responsible for:
-// - Ensuring the main database is closed before calling this method
-// - Reopening the main database after this method returns
-//
-// This ensures database consistency during the copy operation.
+// The only caller (finalizeSnapshotMetadata) does not close the main database
+// before calling this method: the index is copied at dbPath while it is still
+// open, and every step here operates on that copy, never on the live index.
 func (sm *SnapshotManager) ExportSnapshotMetadata(
 	ctx context.Context, dbPath string, snapshotID string,
 ) error {
@@ -415,9 +413,11 @@ func (sm *SnapshotManager) prepareExportDB(
 // uploadSnapshotArtifacts uploads the database backup and blob manifest
 // to remote storage at metadata/<remote-key>/, where remote-key is the
 // double-SHA256 derivation of the snapshot ID (see RemoteSnapshotKey).
-// We never write the human-readable snapshot ID into any unencrypted
-// part of remote storage so a listing of the destination bucket leaks
-// no host, configuration, or scheduling information.
+// The human-readable snapshot ID is never written into an unencrypted part
+// of remote storage, so a plain listing shows only the hashed key, not the
+// hostname or snapshot name. The hash uses no secret, so a guessed hostname
+// and snapshot name can still be confirmed against a listing, and the backup
+// time is public: the manifest carries a plaintext timestamp.
 func (sm *SnapshotManager) uploadSnapshotArtifacts(
 	ctx context.Context, snapshotID string, dbData, manifestData []byte,
 ) error {
@@ -814,10 +814,11 @@ func (sm *SnapshotManager) generateBlobManifest(
 	}
 
 	// Create manifest. SnapshotID in the unencrypted manifest is the
-	// double-SHA256 remote key (see RemoteSnapshotKey), not the human ID,
-	// so neither this field nor the directory name reveals the hostname or
-	// snapshot name. Timestamp below is written in the clear, so the backup
-	// time is observable to anyone who can read the manifest.
+	// double-SHA256 remote key (see RemoteSnapshotKey), not the human ID, so
+	// neither this field nor the directory name spells out the hostname or
+	// snapshot name — but the key uses no secret, so a guessed hostname and
+	// snapshot name can be confirmed. Timestamp below is written in the clear,
+	// so the backup time is observable to anyone who can read the manifest.
 	manifest := &Manifest{
 		SnapshotID:          RemoteSnapshotKey(snapshotID),
 		Timestamp:           time.Now().UTC().Format(time.RFC3339),