feat: add server persistence for rosary streak

- Add RosaryStreak MongoDB model for logged-in users
- Add /api/glaube/rosary-streak GET/POST endpoints
- Sync streak to server when logged in, merge local/server data
- Auto-sync when coming back online (PWA offline support)
- Falls back to localStorage for guests

src/models/RosaryStreak.ts

@@ -0,0 +1,12 @@
+import mongoose from 'mongoose';
+
+const RosaryStreakSchema = new mongoose.Schema(
+  {
+    username: { type: String, required: true, unique: true },
+    length: { type: Number, required: true, default: 0 },
+    lastPrayed: { type: String, default: null } // ISO date string (YYYY-MM-DD)
+  },
+  { timestamps: true }
+);
+
+export const RosaryStreak = mongoose.models.RosaryStreak || mongoose.model("RosaryStreak", RosaryStreakSchema);