feat: ProductMember — team management for product backlogs

- Add ProductMember model (many-to-many User ↔ Product)
- Add productAccessFilter helper (owner OR member OR clause)
- Replace all ownership checks across actions and API routes
- Add addProductMemberAction / removeProductMemberAction / leaveProductAction
- Add TeamManager component in product settings (owner adds/removes Developers)
- Add LeaveProductButton in user settings (member leaves a product team)
- Regenerate Prisma Client after schema migration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/migrations/20260425102419_add_task_implementation_plan/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "tasks" ADD COLUMN     "implementation_plan" TEXT;

prisma/migrations/20260425110346_add_product_members/migration.sql

@@ -0,0 +1,21 @@
+-- CreateTable
+CREATE TABLE "product_members" (
+    "id" TEXT NOT NULL,
+    "product_id" TEXT NOT NULL,
+    "user_id" TEXT NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "product_members_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "product_members_user_id_idx" ON "product_members"("user_id");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "product_members_product_id_user_id_key" ON "product_members"("product_id", "user_id");
+
+-- AddForeignKey
+ALTER TABLE "product_members" ADD CONSTRAINT "product_members_product_id_fkey" FOREIGN KEY ("product_id") REFERENCES "products"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "product_members" ADD CONSTRAINT "product_members_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;