Correct remote layout and privacy docs for hashed snapshot keys (closes #67)

Three documents showed the remote layout with a plaintext snapshot ID as the metadata directory name, and docs/REPOSTRUCTURE.md blamed those IDs for the observable backup time. The store actually names each metadata directory with a one-way hash of the ID, so hostname and snapshot name are not visible; the backup time is, through the plaintext timestamp in the manifest, which is accepted behaviour.

README, ARCHITECTURE.md, docs/DATAMODEL.md and docs/REPOSTRUCTURE.md now show the hashed layout, the derivation is documented once, and the privacy section lists what the unencrypted manifest exposes. Two code comments that claimed the timestamp was hidden are corrected. No behaviour change.

Judgement call: docs/DATAMODEL.md was not named in the issue but had the same error.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

docs/DATAMODEL.md

@@ -194,8 +194,10 @@ After a snapshot is completed:
 2. Clean temporary database to contain only current snapshot data
 3. Export to SQL dump using sqlite3
 4. Compress with zstd and encrypt with age
-5. Upload to S3 as `metadata/{snapshot-id}/db.zst.age`
-6. Generate blob manifest and upload as `metadata/{snapshot-id}/manifest.json.zst`
+5. Upload to S3 as `metadata/{remote-key}/db.zst.age`
+6. Generate blob manifest and upload as `metadata/{remote-key}/manifest.json.zst`
+
+The `{remote-key}` directory name is a one-way hash of the human snapshot ID, so the ID is never written to the store in plaintext; see [REPOSTRUCTURE.md](REPOSTRUCTURE.md#remote-key-derivation).
 
 ### 4. Restore Process