fix(db): add LeadStatus enum, Transaction.buyerId FK, Inquiry compound index

- Convert Lead.status from String to LeadStatus enum (NEW, CONTACTED, QUALIFIED, NEGOTIATING, CONVERTED, LOST) with safe data migration
- Add FK constraint Transaction.buyerId -> User.id to prevent orphaned transactions
- Add compound index on Inquiry(listingId, userId) for duplicate inquiry checks

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/migrations/20260408061200_fix_schema_integrity/migration.sql

@@ -0,0 +1,30 @@
+-- CreateEnum
+CREATE TYPE "LeadStatus" AS ENUM ('NEW', 'CONTACTED', 'QUALIFIED', 'NEGOTIATING', 'CONVERTED', 'LOST');
+
+-- AlterTable: Lead.status from String to LeadStatus enum
+-- Step 1: Add temporary column with enum type
+ALTER TABLE "Lead" ADD COLUMN "status_new" "LeadStatus" NOT NULL DEFAULT 'NEW';
+
+-- Step 2: Migrate existing data (map known string values to enum)
+UPDATE "Lead" SET "status_new" = CASE
+    WHEN "status" = 'new' THEN 'NEW'::"LeadStatus"
+    WHEN "status" = 'contacted' THEN 'CONTACTED'::"LeadStatus"
+    WHEN "status" = 'qualified' THEN 'QUALIFIED'::"LeadStatus"
+    WHEN "status" = 'negotiating' THEN 'NEGOTIATING'::"LeadStatus"
+    WHEN "status" = 'converted' THEN 'CONVERTED'::"LeadStatus"
+    WHEN "status" = 'lost' THEN 'LOST'::"LeadStatus"
+    ELSE 'NEW'::"LeadStatus"
+END;
+
+-- Step 3: Drop old column, rename new column
+ALTER TABLE "Lead" DROP COLUMN "status";
+ALTER TABLE "Lead" RENAME COLUMN "status_new" TO "status";
+
+-- Step 4: Recreate index on Lead.status
+CREATE INDEX "Lead_status_idx" ON "Lead"("status");
+
+-- AddForeignKey: Transaction.buyerId -> User.id
+ALTER TABLE "Transaction" ADD CONSTRAINT "Transaction_buyerId_fkey" FOREIGN KEY ("buyerId") REFERENCES "User"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- CreateIndex: Compound index on Inquiry for duplicate checks
+CREATE INDEX "Inquiry_listingId_userId_idx" ON "Inquiry"("listingId", "userId");