refactor(storage): per-bucket S3 profiles instead of s3+<name> scheme

[papaharry]

Summary

Reworks S3-compatible multi-backend support per review feedback: instead of embedding a config label in the URI scheme (s3+<name>://), which gets persisted into the metastore, route by bucket name via per-bucket profiles under storage.s3.profiles.<bucket>. URIs stay canonical s3://bucket/path — nothing synthetic is persisted; routing is resolved at runtime from node-local config.

Why

The previous s3+<name>:// design baked a config-time label into durable metastore state (the index_uri and per-split metadata). That made profiles un-renameable, tied environments to a shared label convention, and required teaching the Uri type a non-canonical scheme. Keying on bucket name (already intrinsic to the object location) avoids all of that.

Changes

• quickwit-common/uri.rs: revert all s3+<name> scheme handling to upstream — Uri stays canonical.
• quickwit-config: named → profiles (keyed by bucket), NamedS3StorageConfig → S3ProfileConfig, is_named_backend → is_profile. Drop URL-scheme name validation (a bucket name isn't a scheme).
• quickwit-storage: resolver extracts the bucket via parse_s3_uri and looks it up in profiles — match → that profile's config + per-bucket client cache; no match → primary backend.
• docs: rewrite as "Per-bucket S3 profiles", including the bucket-name uniqueness caveat (a bucket name maps to exactly one backend).

Trade-off

Bucket-keyed routing cannot disambiguate the same bucket name across two different endpoints (S3-compatible namespaces are per-endpoint). Documented; acceptable since bucket names are under our control.

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy -p quickwit-config -p quickwit-storage --all-features --tests (clean)
• cargo test -p quickwit-config storage_config (14 passed) + -p quickwit-common uri (9 passed)
• End-to-end through the REST stack: ingest → split → upload → publish → search, validated against an in-cluster SeaweedFS profile and an external OVH profile (own credentials, routed purely by bucket name), plus unlisted-bucket fallback to the primary backend.

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