feat: add GitHub activity intelligence engine with mobile UI(fixes #132)

.env.example

@@ -26,3 +26,7 @@ MOBILE_REDIRECT_URI=devcard://oauth/callback
 # ─── Server ───
 PORT=3000
 NODE_ENV=development
+
+# ─── AI (optional — enables AI-generated developer summaries) ───
+# Get a free key at https://aistudio.google.com/app/apikey
+GEMINI_API_KEY=

apps/backend/prisma/migrations/20260519000000_github_insights_cache/migration.sql

@@ -0,0 +1,17 @@
+-- CreateTable
+CREATE TABLE "github_insights_cache" (
+    "id" TEXT NOT NULL,
+    "user_id" TEXT NOT NULL,
+    "payload" JSONB NOT NULL,
+    "expires_at" TIMESTAMP(3) NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "github_insights_cache_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "github_insights_cache_user_id_key" ON "github_insights_cache"("user_id");
+
+-- AddForeignKey
+ALTER TABLE "github_insights_cache" ADD CONSTRAINT "github_insights_cache_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;

apps/backend/prisma/schema.prisma

@@ -29,6 +29,7 @@ model User {
   ownedViews    CardView[]     @relation("cardOwner")
   viewedCards   CardView[]     @relation("cardViewer")
   followLogs    FollowLog[]
+  githubInsightsCache GitHubInsightsCache?
 
   @@unique([provider, providerId])
   @@map("users")
@@ -124,3 +125,16 @@ model FollowLog {
 
   @@map("follow_logs")
 }
+
+model GitHubInsightsCache {
+  id        String   @id @default(uuid())
+  userId    String   @unique @map("user_id")
+  payload   Json                              // serialized GitHubInsights
+  expiresAt DateTime @map("expires_at")
+  createdAt DateTime @default(now()) @map("created_at")
+  updatedAt DateTime @updatedAt @map("updated_at")
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@map("github_insights_cache")
+}