feat(t-paliad-209): rename DeadlineRule.Code → SubmissionCode across Go layer

Workstream B Go sweep — matches mig 098. Every place the deadline-rules
service reads/writes the per-rule identifier now uses the new column
name and the new struct field. Distinct from rule_code (legal citation)
and from proceeding_types.code (the proceeding's 3-segment code).

Touch points:
- models.DeadlineRule.Code → SubmissionCode (db + json tags renamed
  in lockstep — JSON contract `submission_code` is the new shape).
- deadline_rule_service: ruleColumns SELECT list updated.
- rule_editor_service: CreateRuleInput.Code → SubmissionCode (json tag
  too), INSERT + CloneAsDraft SELECT updated.
- projection_service: lookupRuleByCode → lookupRuleBySubmissionCode
  (SQL WHERE clause + error message); every r.Code / parent.Code /
  rule.Code / first.Code / src.rule.Code read renamed.
- fristenrechner: r.Code / prev.Code / rule.Code reads renamed in
  Calculate (parent-anchor + override-key + computed-by-code map) and
  in CalculateRule's LocalCode emission; the proceeding-code+submission-
  code resolver query uses `submission_code = $2`.
- event_trigger_service / deadline_calculator: r.Code reads renamed.

UIDeadline.Code (the calculator's wire response) is unchanged — that
field is a separate API contract pointing at the same value; renaming
it would force every frontend deadline-renderer through a contract
break that isn't part of this workstream.

Test fixtures updated to the new SubmissionCode field name; live-DB
tests updated to the post-mig-098 prefixed values (`inf.sod` →
`upc.inf.cfi.sod` etc.). New submission_codes_shape_test asserts
every active+published row matches the 4+-segment proceeding-prefixed
shape (sibling of TestProceedingCodeShape; mirrors mig 098 §6.1).

go build ./... clean. go test ./internal/... green.

internal/models/models.go

@@ -467,7 +467,7 @@ type DeadlineRule struct {
 	ID               uuid.UUID  `db:"id"                 json:"id"`
 	ProceedingTypeID *int       `db:"proceeding_type_id" json:"proceeding_type_id,omitempty"`
 	ParentID         *uuid.UUID `db:"parent_id"          json:"parent_id,omitempty"`
-	Code             *string    `db:"code"               json:"code,omitempty"`
+	SubmissionCode   *string    `db:"submission_code"    json:"submission_code,omitempty"`
 	Name             string     `db:"name"               json:"name"`
 	NameEN           string     `db:"name_en"            json:"name_en"`
 	Description      *string    `db:"description"        json:"description,omitempty"`