perf(blob): one manifest query in read_blob_bytes instead of two

read_blob_bytes read the same storage.chunk_manifests PK row twice per
full-blob read — blob_size (SELECT total_size) then read_blob_stream
(SELECT chunk_hashes) — even though both columns live in one row. Fold
them into a single `SELECT chunk_hashes, total_size` and share the chunk
stream builder via a new stream_chunks helper. The legacy (no-manifest)
path is unchanged. Output is identical; the read just costs one fewer DB
round-trip.

Benchmark (examples/bench_blob_manifest.rs, isolates the manifest lookup
against the real Postgres): ~1.9x throughput and p50/p99 roughly halved on
that sub-step; the win is the removed round-trip under pool pressure during
upload bursts. Note this is the manifest sub-step only — end-to-end
read_blob_bytes is dominated by the actual chunk reads, and it is a
background path (thumbnail generation / EXIF / indexing), not normal gallery
serving. Methodology + honest framing in benches/BLOB-MANIFEST.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -168,6 +168,12 @@ name = "bench_db_pool"
 path = "examples/bench_db_pool.rs"
 required-features = ["bench"]
 
+# Blob-manifest read round-trip benchmark: OLD 2 queries vs NEW 1 (needs dev Postgres).
+[[example]]
+name = "bench_blob_manifest"
+path = "examples/bench_blob_manifest.rs"
+required-features = ["bench"]
+
 # ACL owner-cache benchmark — owner query vs moka hit (needs the dev Postgres up).
 [[example]]
 name = "bench_owner_cache"