feat: add opt-in writable HTTP tables

[sabino]

What changed

• implement native PostgreSQL INSERT, UPDATE, and DELETE callbacks
• map writes to explicit POST/PUT/PATCH/DELETE endpoints while keeping every table read-only by default
• support typed-column whitelists plus schema-flexible attrs and merge JSONB request bodies
• URL-encode row identities and reject unsafe or ambiguous path templates
• retry only HTTP methods with idempotent semantics; POST and PATCH make one attempt
• exercise POST, PATCH, PUT, DELETE, 204 responses, body filtering, and dynamic JSONB with real PostgreSQL 14-18 integration tests
• document HTTP transaction boundaries and the current lack of RETURNING

Why

The native FDW could scan read APIs but its framework modification hooks used their no-op defaults. Explicit mutation capabilities turn PostgreSQL into a useful read/write interface for compatible APIs without making accidental writes possible.

The supabase-wrappers dependency remains only a native Rust adapter for PostgreSQL callbacks. This change does not introduce a hosted Supabase dependency or a Wasm execution path.

Validation

• cargo fmt --all -- --check
• python3 -m py_compile tests/mock_api.py
• git diff --check
• full PostgreSQL 14-18 native matrix delegated to this PR's GitHub Actions to avoid a large local build under current disk pressure