feat: add post3 s3 proxy for postgresql

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

todos/POST3-007-integration-tests.md

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+# POST3-007: Integration tests with aws-sdk-s3
+
+**Status:** Done
+**Priority:** P1
+**Blocked by:** POST3-006
+
+## Description
+
+End-to-end integration tests using the official AWS S3 Rust SDK to validate the full stack.
+
+## Acceptance Criteria
+
+- [ ] `tests/common/mod.rs` — TestServer helper:
+  - Starts server on ephemeral port (port 0)
+  - Configures aws-sdk-s3 with force_path_style, dummy creds, custom endpoint
+  - Cleans database between tests
+- [ ] Test: create + list buckets
+- [ ] Test: head bucket (exists + not exists)
+- [ ] Test: delete bucket
+- [ ] Test: put + get small object (body roundtrip)
+- [ ] Test: put large object (5 MiB, verify chunked storage + reassembly)
+- [ ] Test: head object (size, etag, content-type)
+- [ ] Test: delete object (verify 404 after)
+- [ ] Test: list objects v2 with prefix filter
+- [ ] Test: overwrite object (verify latest version)
+- [ ] Test: user metadata roundtrip (x-amz-meta-* headers)
+- [ ] All tests pass with `cargo nextest run`