feat: add post3 s3 proxy for postgresql

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/POST3-006-s3-handlers.md

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+# POST3-006: S3 bucket and object handlers
+
+**Status:** Done
+**Priority:** P1
+**Blocked by:** POST3-005
+
+## Description
+
+Implement all S3 HTTP request handlers that bridge the S3 REST API to the core Store API.
+
+## Acceptance Criteria
+
+### Bucket handlers (`s3/handlers/buckets.rs`)
+- [ ] CreateBucket — PUT /{bucket} → 200 + Location header
+- [ ] HeadBucket — HEAD /{bucket} → 200 or 404
+- [ ] DeleteBucket — DELETE /{bucket} → 204 (409 if not empty)
+- [ ] ListBuckets — GET / → 200 + XML
+
+### Object handlers (`s3/handlers/objects.rs`)
+- [ ] PutObject — PUT /{bucket}/{*key} → 200 + ETag header; reads x-amz-meta-* from request headers
+- [ ] GetObject — GET /{bucket}/{*key} → 200 + body + ETag + Content-Type + Content-Length + Last-Modified + x-amz-meta-* headers
+- [ ] HeadObject — HEAD /{bucket}/{*key} → 200 + metadata headers (no body)
+- [ ] DeleteObject — DELETE /{bucket}/{*key} → 204
+- [ ] ListObjectsV2 — GET /{bucket}?list-type=2 → 200 + XML
+
+### Error handling
+- [ ] NoSuchBucket → 404 + XML error
+- [ ] NoSuchKey → 404 + XML error
+- [ ] BucketAlreadyOwnedByYou → 409 + XML error
+- [ ] BucketNotEmpty → 409 + XML error