feat: add post3 s3 proxy for postgresql

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

todos/POST3-003-repository-layer.md

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+# POST3-003: Repository layer and Store API
+
+**Status:** Done
+**Priority:** P0
+**Blocked by:** POST3-002
+
+## Description
+
+Implement the repository layer (raw SQL CRUD for each table) and the high-level Store API that orchestrates them with transactions and chunking logic.
+
+## Acceptance Criteria
+
+- [ ] `repositories/buckets.rs` — create, get_by_name, list, delete, is_empty
+- [ ] `repositories/objects.rs` — upsert, get, delete, list (with prefix + pagination)
+- [ ] `repositories/blocks.rs` — insert_block, get_all_blocks (ordered by block_index)
+- [ ] `repositories/metadata.rs` — insert_batch, get_all (for an object_id)
+- [ ] `store.rs` — Store struct with all public methods:
+  - create_bucket, head_bucket, delete_bucket, list_buckets
+  - put_object (chunking + MD5 ETag + metadata, all in a transaction)
+  - get_object (reassemble blocks + fetch metadata)
+  - head_object, delete_object, list_objects_v2
+  - get_object_metadata
+- [ ] put_object correctly splits body into 1 MiB blocks
+- [ ] get_object correctly reassembles blocks in order
+- [ ] Overwriting an object deletes old blocks+metadata via CASCADE
+- [ ] `cargo check -p post3` passes