Make migrations idempotent and add missing tables

- Make all pending migrations idempotent (safe to re-run)
- Disable decouple_projects_from_rounds migration (schema not changed)
- Add ProjectTag table for AI tagging
- Add AssignmentJob table for AI assignment progress

On server deployment, run: npx prisma migrate deploy

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

prisma/migrations/20260203000000_add_ai_usage_log/migration.sql

@@ -1,5 +1,5 @@
 -- CreateTable
-CREATE TABLE "AIUsageLog" (
+CREATE TABLE IF NOT EXISTS "AIUsageLog" (
     "id" TEXT NOT NULL,
     "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
     "userId" TEXT,
@@ -21,13 +21,7 @@ CREATE TABLE "AIUsageLog" (
 );
 
 -- CreateIndex
-CREATE INDEX "AIUsageLog_userId_idx" ON "AIUsageLog"("userId");
-
--- CreateIndex
-CREATE INDEX "AIUsageLog_action_idx" ON "AIUsageLog"("action");
-
--- CreateIndex
-CREATE INDEX "AIUsageLog_createdAt_idx" ON "AIUsageLog"("createdAt");
-
--- CreateIndex
-CREATE INDEX "AIUsageLog_model_idx" ON "AIUsageLog"("model");
+CREATE INDEX IF NOT EXISTS "AIUsageLog_userId_idx" ON "AIUsageLog"("userId");
+CREATE INDEX IF NOT EXISTS "AIUsageLog_action_idx" ON "AIUsageLog"("action");
+CREATE INDEX IF NOT EXISTS "AIUsageLog_createdAt_idx" ON "AIUsageLog"("createdAt");
+CREATE INDEX IF NOT EXISTS "AIUsageLog_model_idx" ON "AIUsageLog"("model");