Mark a snapshot complete only after its metadata export succeeds (closes #177)

finalizeSnapshotMetadata marked the snapshot complete and then exported its metadata. A crash after completion but before/during the export left the local index showing the snapshot complete while the destination had no manifest or database, and PruneDatabase (which drops only NULL completed_at rows) kept it: a silently unrestorable snapshot.

Reorder so completion is recorded last. CompleteSnapshot is split into PopulateSnapshotBlobs (before the export) and MarkSnapshotComplete (after it). An interrupted export now leaves the snapshot incomplete, so the next run PruneDatabase drops it and re-backs-up the data; the reverse tiny window leaves a restorable snapshot the index reports honestly as remote-only. Update REPOSTRUCTURE.md guarantee 4 and the ARCHITECTURE.md flow. Add a fault-injection test driving the full create path.

Model: opus-4-8

ARCHITECTURE.md

@@ -284,8 +284,10 @@ Manages snapshot lifecycle and metadata export.
 
 Key methods:
 - `CreateSnapshot(ctx, hostname, version, commit)` → Create snapshot record
-- `CompleteSnapshot(ctx, snapshotID)` → Mark snapshot complete
+- `PopulateSnapshotBlobs(ctx, snapshotID)` → Record every blob the snapshot references
 - `ExportSnapshotMetadata(ctx, dbPath, snapshotID)` → Export to S3
+- `MarkSnapshotComplete(ctx, snapshotID)` → Record completion, only after a successful export
+- `CompleteSnapshot(ctx, snapshotID)` → Convenience: populate blobs, then mark complete (no export between)
 
 ### `internal/database`
 SQLite database for local index. Single-writer mode for thread safety.
@@ -335,16 +337,18 @@ CreateSnapshot(opts)
     │
     ├─► SnapshotManager.UpdateSnapshotStatsExtended()
     │
-    ├─► SnapshotManager.CompleteSnapshot()
+    ├─► SnapshotManager.PopulateSnapshotBlobs()   // record referenced blobs
     │
-    └─► SnapshotManager.ExportSnapshotMetadata()
-            │
-            ├─► Copy database to temp file
-            ├─► Clean to only current snapshot data (VACUUM)
-            ├─► Compress binary SQLite with zstd
-            ├─► Encrypt with age
-            ├─► Upload db.zst.age to storage
-            └─► Upload manifest.json.zst to storage
+    ├─► SnapshotManager.ExportSnapshotMetadata()
+    │       │
+    │       ├─► Copy database to temp file
+    │       ├─► Clean to only current snapshot data (VACUUM)
+    │       ├─► Compress binary SQLite with zstd
+    │       ├─► Encrypt with age
+    │       ├─► Upload db.zst.age to storage
+    │       └─► Upload manifest.json.zst to storage
+    │
+    └─► SnapshotManager.MarkSnapshotComplete()   // only after the export succeeds
 ```
 
 ## Deduplication Strategy