feat: add post3 s3 proxy for postgresql

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

todos/POST3-002-schema-models-errors.md

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+# POST3-002: Database schema, models, and error types
+
+**Status:** Done
+**Priority:** P0
+**Blocked by:** POST3-001
+
+## Description
+
+Define the PostgreSQL schema (buckets, objects, object_metadata, blocks), create Rust model types with sqlx::FromRow, and define the Post3Error enum.
+
+## Acceptance Criteria
+
+- [ ] `crates/post3/migrations/20260226000001_initial.sql` with all 4 tables + indexes
+- [ ] `crates/post3/src/models.rs` — BucketRow, ObjectRow, BlockRow, MetadataEntry, ObjectInfo, ListObjectsResult
+- [ ] `crates/post3/src/error.rs` — Post3Error enum (BucketNotFound, BucketAlreadyExists, ObjectNotFound, BucketNotEmpty, Database, Other)
+- [ ] Migration runs successfully against PostgreSQL
+- [ ] `cargo check -p post3` passes
+
+## Schema Details
+
+- `buckets` — id (UUID PK), name (TEXT UNIQUE), created_at
+- `objects` — id (UUID PK), bucket_id (FK CASCADE), key, size, etag, content_type, created_at; unique on (bucket_id, key)
+- `object_metadata` — id (UUID PK), object_id (FK CASCADE), meta_key, meta_value; unique on (object_id, meta_key)
+- `blocks` — id (UUID PK), object_id (FK CASCADE), block_index, data (BYTEA), block_size; unique on (object_id, block_index)