Protocol for introspecting different schema sources.

This protocol provides a unified interface for discovering database schema information from different sources:

• Ecto schema modules (via __schema__/1 callbacks)
• Direct PostgreSQL connections (via Postgrex and system catalogs)
• Future: Other database types (MySQL, SQLite, etc.)

Usage

# Ecto schema
{:ok, metadata} = SelectoMix.Introspector.introspect(MyApp.User, [])

# Postgrex connection
{:ok, conn} = Postgrex.start_link(...)
{:ok, metadata} = SelectoMix.Introspector.introspect(
  {:postgrex, conn, "users"},
  []
)

Metadata Format

All introspectors return a standardized metadata map:

%{
  table_name: "users",
  schema: "public",
  fields: [:id, :name, :email, :inserted_at, :updated_at],
  field_types: %{
    id: :integer,
    name: :string,
    email: :string,
    inserted_at: :naive_datetime,
    updated_at: :naive_datetime
  },
  primary_key: :id,  # or [:id, :tenant_id] for composite
  associations: %{
    posts: %{
      type: :has_many,
      ...
    },
    profile: %{
      type: :has_one,
      ...
    }
  },
  columns: %{
    id: %{type: :integer, nullable: false, ...},
    name: %{type: :string, nullable: false, ...},
    ...
  },
  source: :ecto  # or :postgres, :mysql, etc.
}