feat(db): add return_match table for #338

New model models.ReturnMatch backing the SYB↔yeeke return matching
feature: matched/confirmed/cancelled status, nullable
ActiveSYBProductID/ActiveYeekeReturnItemID columns (same pattern as
YeekeSyncRun.ActiveSlot) each carrying a unique index so only one
active match can occupy either side at a time; cancel clears both to
free the slot for a rematch. Registered in migrations.MigratedModels().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTDbDcwbDw1TSAcE6wfh2F

server/app/goauto/migrations/migrate.go

@@ -79,6 +79,7 @@ func MigratedModels() []any {
 		&models.PDDProductReplacement{},
 		&models.PDDProductReplacementItem{},
 		&models.PDDProductReplacementWorkerLease{},
+		&models.ReturnMatch{},
 	}
 }
 

server/app/goauto/models/return_match.go

@@ -0,0 +1,71 @@
+package models
+
+import "time"
+
+// #338: ReturnMatch links one syb_product row to one yeeke_return_item row so
+// the return can be reused instead of buying the item again on PDD. A match
+// is "active" (still blocking purchase creation and occupying the return)
+// while Status is matched or confirmed; cancelling clears both Active*
+// columns so the unique index frees up both sides for a future rematch
+// (issue #338, business rule: 取消匹配后再次点击「匹配退货」若配回同一对，允许).
+//
+// ActiveSYBProductID/ActiveYeekeReturnItemID follow the same nullable-unique-
+// column pattern already used by YeekeSyncRun.ActiveSlot in this codebase:
+// MySQL unique indexes allow any number of NULL rows, so only the currently
+// active match for a given side occupies the slot, and a concurrent second
+// insert for the same side fails the unique constraint instead of double-
+// booking it.
+const (
+	ReturnMatchStatusMatched   = "matched"
+	ReturnMatchStatusConfirmed = "confirmed"
+	ReturnMatchStatusCancelled = "cancelled"
+)
+
+type ReturnMatch struct {
+	ID uint64 `json:"id" gorm:"primaryKey;autoIncrement"`
+
+	SYBProductID      uint64 `json:"sybProductId" gorm:"not null;index"`
+	YeekeReturnItemID uint64 `json:"yeekeReturnItemId" gorm:"not null;index"`
+
+	// ActiveSYBProductID/ActiveYeekeReturnItemID mirror the ID columns above
+	// while Status is matched/confirmed, and are cleared to NULL on cancel.
+	ActiveSYBProductID      *uint64 `json:"-" gorm:"uniqueIndex:ux_return_match_active_syb"`
+	ActiveYeekeReturnItemID *uint64 `json:"-" gorm:"uniqueIndex:ux_return_match_active_return"`
+
+	Status string `json:"status" gorm:"size:16;not null;index;check:ck_return_match_status,status IN ('matched','confirmed','cancelled')"`
+
+	// Snapshots of the normalized spec text on both sides at match time, kept
+	// for audit and for the compare screen even if the source rows change
+	// later.
+	SYBSpecText   string `json:"sybSpecText" gorm:"size:255;not null;default:''"`
+	YeekeSpecText string `json:"yeekeSpecText" gorm:"size:255;not null;default:''"`
+	NormalizedKey string `json:"normalizedKey" gorm:"size:255;not null;default:'';index"`
+
+	// PreviousProcessStage records what the SYB product's computed process
+	// stage was immediately before this match, so cancel can report/restore
+	// context; the actual restore is a pure recomputation (no stage is
+	// stored on syb_product), this is for audit/display only.
+	PreviousProcessStage string `json:"previousProcessStage" gorm:"size:32;not null;default:''"`
+
+	DestroyDeadlineSnapshot *time.Time `json:"destroyDeadlineSnapshot,omitempty"`
+
+	Remark string `json:"remark" gorm:"size:500;not null;default:''"`
+
+	MatchedBy   string     `json:"matchedBy" gorm:"size:64;not null;default:''"`
+	MatchedAt   time.Time  `json:"matchedAt"`
+	ConfirmedBy string     `json:"confirmedBy,omitempty" gorm:"size:64;not null;default:''"`
+	ConfirmedAt *time.Time `json:"confirmedAt,omitempty"`
+	CancelledBy string     `json:"cancelledBy,omitempty" gorm:"size:64;not null;default:''"`
+	CancelledAt *time.Time `json:"cancelledAt,omitempty"`
+
+	CreatedAt time.Time `json:"createdAt"`
+	UpdatedAt time.Time `json:"updatedAt"`
+}
+
+func (ReturnMatch) TableName() string { return "return_match" }
+
+// IsActive reports whether the match currently blocks purchase creation and
+// occupies its return item.
+func (m ReturnMatch) IsActive() bool {
+	return m.Status == ReturnMatchStatusMatched || m.Status == ReturnMatchStatusConfirmed
+}