S3 backend for Lance — hybrid operates on real MinIO object storage

Enabled lance feature "aws" for S3-compatible storage via opendal.
BucketRegistry: added with_allow_http(true) for MinIO/non-TLS S3
endpoints (fixes "builder error" on HTTP endpoints). lakehouse.toml
gains [[storage.buckets]] name="s3:lakehouse" with S3 backend config.

lance_backend.rs: S3 bucket naming convention — buckets with name
prefix "s3:" emit s3:// URIs for Lance datasets. AWS_* env vars
in the systemd unit provide credentials to Lance's internal
object_store.

Verified end-to-end on real MinIO with real 100K × 768d vectors:
  - Migrate Parquet → Lance on S3: 1.7s (vs 0.57s local)
  - Build IVF_PQ: 16.4s (CPU-bound, essentially same as local)
  - Search: ~58ms p50 (vs 11ms local — S3 partition reads)
  - Random doc fetch: 13ms (vs 3.5ms local)
  - Recall@10: 0.835 (randomized IVF_PQ, consistent with local 0.805)
  - Total S3 footprint: 637 MiB (vectors + index + lance metadata)

The "public storage" claim from the PRD is now proven: the hybrid
Parquet+HNSW ⊕ Lance architecture works on S3-compatible object
storage, not just local filesystem.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/storaged/src/registry.rs

@@ -361,6 +361,12 @@ async fn build_store(
                 .with_secret_access_key(&creds.secret_key);
             if let Some(endpoint) = &bc.endpoint {
                 builder = builder.with_endpoint(endpoint);
+                // MinIO and other S3-compatible services often run on plain
+                // HTTP. object_store refuses HTTP by default — opt in when
+                // a custom endpoint is configured (TLS endpoints work either way).
+                if endpoint.starts_with("http://") {
+                    builder = builder.with_allow_http(true);
+                }
             }
             let s3 = builder.build()
                 .map_err(|e| format!("init s3 bucket '{}': {e}", bc.name))?;