feat: add post3 s3 proxy for postgresql

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

todos/POST3-008-client-sdk.md

@@ -0,0 +1,26 @@
+# POST3-008: Client SDK crate
+
+**Status:** Done
+**Priority:** P0
+**Blocked by:** —
+
+## Description
+
+Create a `crates/post3-sdk/` client crate that wraps `aws-sdk-s3` with post3-specific defaults.
+
+## What was built
+
+- [x] `crates/post3-sdk/Cargo.toml` — depends on aws-sdk-s3, aws-credential-types, aws-types, aws-config
+- [x] `Post3Client` struct wrapping `aws_sdk_s3::Client`
+- [x] `Post3Client::new(endpoint_url)` — builds client with force_path_style, dummy creds, us-east-1
+- [x] `Post3Client::builder()` — for advanced config (custom creds, region, etc.)
+- [x] Re-exports: `aws_sdk_s3` and `bytes`
+- [x] Convenience methods:
+  - `create_bucket(name)`, `head_bucket(name)`, `delete_bucket(name)`, `list_buckets()`
+  - `put_object(bucket, key, body: impl AsRef<[u8]>)`
+  - `get_object(bucket, key)` → `Result<Bytes>`
+  - `head_object(bucket, key)` → `Result<Option<ObjectInfo>>`
+  - `delete_object(bucket, key)`
+  - `list_objects(bucket, prefix)` → `Result<Vec<ObjectInfo>>`
+- [x] `inner()` access to `aws_sdk_s3::Client`
+- [x] Unit tests + doc-tests pass