Replace string-based ApplicationStatus with a closed iota-based enum

[mushrafmim]

Current Limitation

ApplicationStatus is represented as a bare Go string everywhere it's touched — ApplicationRecord.Status and Application.Status in internal/application/store.go/service.go, and mirrored again in ConsignmentRecord.Status (internal/consignment/store.go). Nothing in code enforces the closed set; only docs/task-config-reference.md's "Application status lifecycle" section documents it as the target contract (PENDING, FEEDBACK_REQUESTED, APPROVED, REJECTED).

Concretely, ReviewApplication (internal/application/service.go) still defaults an unresolved review outcome to the literal "DONE" — a value outside the documented closed set — instead of failing closed. Status comparisons throughout the store layer (store.go) are raw string literals ("PENDING", "FEEDBACK_REQUESTED", ...) with no compiler-checked exhaustiveness, so a typo or a new ad-hoc status string would silently pass through.

Suggested Improvement

Introduce an iota-based ApplicationStatus type (in internal/application) with exactly the 4 canonical values (StatusPending, StatusFeedbackRequested, StatusApproved, StatusRejected), implementing String(), MarshalJSON/UnmarshalJSON, and Value()/Scan() so the JSON wire format and the varchar(50) DB column are unaffected — only the in-memory representation changes from string comparisons to a closed, typed enum.

Replace the raw string literals in store.go, service.go, and the mirrored status on consignment.Store with the typed constants. As part of this, retire the "DONE" fallback in ReviewApplication per the target contract already documented in docs/task-config-reference.md#application-status-lifecycle-canonical-applies-to-every-task — an outcome that doesn't resolve via statusMap/autoApprove should reject the review request (400) rather than silently storing a status outside the closed set.

Stretch goal (can be split further if needed): extend TaskConfig.Validate to reject behavior.statusMap values outside {APPROVED, REJECTED, FEEDBACK_REQUESTED} at config load time, closing the other half of the same target contract.

Version

28356cb (branch impr/244-improve-task-config-validation, PR #245)

Additional Context

Follow-up to #244 / PR #245, which tightened TaskConfig validation (forms.review/behavior required, Behavior converted to a value type) but deliberately left the ApplicationStatus representation and the statusMap value contract untouched to keep that PR scoped to config-shape validation. This issue tracks that remaining half.

Relevant files:

• internal/application/store.go
• internal/application/service.go
• internal/consignment/store.go
• docs/task-config-reference.md (canonical status lifecycle table)