Record-transformation and value-preparation helpers shared across the import paths in ConnectionNotifier.

Three concerns:

1. value/1 and value/3 — convert JSON-transported scalars back into Postgrex-ready terms. ISO8601 strings become DateTime / Date / Time. Numeric strings are parsed only when the target column is numeric (the 3-arity form): a %Decimal{} for numeric/decimal, a float for double precision/real. The broad 1-arity form (Decimal only) is kept for the PK/unique-key lookup paths where values are known keys, not free-text.
2. numeric_column_types/1 — maps each numeric column of a table to :decimal or :float. Cached once per table-import by the importer; empty map on error (safe "don't coerce" fallback).
3. Record field helpers (get_field/2, put_field/3, drop_field/2, normalize_keys/1) — records arrive with either string or atom keys depending on whether they came through JSON or internal code. These helpers access/modify fields uniformly without creating atoms from untrusted data.

Extracted from ConnectionNotifier in 2026-04. All functions are pure or depend only on SchemaInspector (for column introspection) — none touch HTTP, the socket, or the websocket client.