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. Decimal-like strings become %Decimal{} only when the target column is numeric (the 3-arity form) — the broad 1-arity form is kept for the PK/unique-key lookup paths where values are known keys, not free-text.
2. numeric_columns/1 — returns the list of column names whose type is numeric/decimal/double precision/real for a given table. Used by the importer to scope decimal-string detection. Cached once per table-import; empty list 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.