[DwC export]: Extend `spqueryfield` for DwC term mapping

[grantfitzsimmons]

Goal

Extend the spqueryfield table with a Term column to store the DwC IRI or term name associated with an output field. Include stub columns for V2 static value support (#7720).

Background

Term-to-field associations must be stored at the field level so each output column has its own concept mapping. The Term column is the primary V1 addition; IsStatic and StaticValue are deferred to V2.

Schema Addition

Field	Type	Notes
Term	VarChar (nullable)	IRI or term name; null = unmapped
IsStatic	Boolean (V2)	Default false
StaticValue	Text (V2)	Nullable

Acceptance Criteria

• Term column added to spqueryfield in a migration (nullable, default null).
• Existing spqueryfield rows are unaffected (null Term = standard query field, no mapping).
• ORM model updated to expose the Term field.
• IsStatic and StaticValue columns added as nullable stubs (no UI wired for V2 yet).
• All export generation code reads Term from spqueryfield to set CSV column headers.