Make crate idiomatic, production-ready, and easy to use

[kacy]

Summary

Bumps version to 0.3.0 with the following changes:

• Unified error type: Replace LogoutError and DbError with a single thiserror-derived Error enum (Config, Request, Serialization, Jwt, AuthRequired, Api)
• Fallible constructor: Supabase::new() returns Result, validating that url and api_key are present
• Typed auth responses: Auth methods return AuthResponse or EmptyResponse instead of raw reqwest::Response, with automatic non-2xx → Error::Api conversion
• Header deduplication: Private auth_post() helper eliminates repeated header setup across 9 auth methods
• DB error handling: execute() checks HTTP status and returns Error::Api for non-2xx; new execute_and_parse::<T>() for automatic JSON deserialization
• Dependency cleanup: Remove unused time, move tokio to dev-deps, add thiserror = "2"
• Ergonomic re-exports: Error, AuthResponse, Claims, QueryBuilder available at crate root
• Documentation: Crate-level doc comment with quick start example; crates.io keywords/categories
• #[must_use] on QueryBuilder to warn if .execute() is forgotten
• Version bump from 0.2.0 → 0.3.0

Breaking changes

Change	Migration
Supabase::new() returns Result	Add ? or .unwrap()
Auth methods return AuthResponse/EmptyResponse	Access fields directly
logout() returns single Result	Remove nested match
DbError removed	Use supabase_rust::Error
LogoutError removed	Use Error::AuthRequired
tokio moved to dev-deps	Users provide their own async runtime

Test plan

• cargo build --lib compiles cleanly
• cargo test — all 27 tests pass
• cargo doc --no-deps — docs generate without warnings