fix(functions): add server-side throttle to gentle_reminders callable

functions/src/notifications/sendGentleReminderCallable.ts

@@ -1,13 +1,21 @@
 import * as functions from 'firebase-functions'
 import * as admin from 'firebase-admin'
 
+const GENTLE_REMINDER_MAX_PER_HOUR = 5
+const GENTLE_REMINDER_WINDOW_MS = 60 * 60 * 1000 // 1 hour
+
 /**
  * Sends a gentle nudge from one partner to the other when the caller has
  * already answered today's question but the partner hasn't.
  *
- * Rate limit: one reminder per couple per calendar day (UTC). The lock is
- * stored in couples/{coupleId}/gentle_reminders/{date} so it survives
- * function restarts and is visible to both partners.
+ * Rate limits:
+ *  - Per-user: max 5 gentle reminders per rolling hour. Guarded by a server-side
+ *    transaction on `rate_limits/{uid}_gentle_reminder` so malicious clients
+ *    cannot bypass it by calling the callable in a loop. The Android-side
+ *    NotificationRateLimiter remains for UX but is not the authoritative guard.
+ *  - Per-couple: one reminder per couple per calendar day (UTC). The lock is
+ *    stored in couples/{coupleId}/gentle_reminders/{date} so it survives
+ *    function restarts and is visible to both partners.
  *
  * The notification is both an FCM push (for the system tray) and an entry in
  * the partner's notification_queue (for in-app display).
@@ -39,7 +47,56 @@ export const sendGentleReminderCallable = functions.https.onCall(async (_data, c
     throw new functions.https.HttpsError('failed-precondition', 'No partner found.')
   }
 
-  // ── 2. Rate limit: one per couple per day ────────────────────────────────
+  // ── 2. Server-side per-user throttle: 5 per hour (rolling window) ────────
+
+  const now = admin.firestore.Timestamp.now()
+  const rateLimitRef = db.collection('rate_limits').doc(`${callerId}_gentle_reminder`)
+
+  const throttleResult = await db.runTransaction(async (tx) => {
+    const snap = await tx.get(rateLimitRef)
+    const data = snap.data()
+
+    let windowStart: admin.firestore.Timestamp
+    let count: number
+
+    if (!snap.exists || !data) {
+      windowStart = now
+      count = 0
+    } else {
+      windowStart = data.windowStart as admin.firestore.Timestamp
+      count = (typeof data.count === 'number' ? data.count : 0)
+
+      const elapsedMs = now.toMillis() - windowStart.toMillis()
+      if (elapsedMs >= GENTLE_REMINDER_WINDOW_MS) {
+        // Rolling window has expired; start a fresh one.
+        windowStart = now
+        count = 0
+      }
+    }
+
+    if (count >= GENTLE_REMINDER_MAX_PER_HOUR) {
+      const retryAfterMs = GENTLE_REMINDER_WINDOW_MS - (now.toMillis() - windowStart.toMillis())
+      const retryAfterMinutes = Math.max(1, Math.ceil(retryAfterMs / 60_000))
+      return { allowed: false, retryAfterMinutes }
+    }
+
+    tx.set(rateLimitRef, {
+      count: count + 1,
+      windowStart,
+      updatedAt: now,
+    }, { merge: true })
+
+    return { allowed: true, count: count + 1, windowStart }
+  })
+
+  if (!throttleResult.allowed) {
+    throw new functions.https.HttpsError(
+      'resource-exhausted',
+      `Too many gentle reminders. Try again in ${throttleResult.retryAfterMinutes} minutes.`
+    )
+  }
+
+  // ── 3. Rate limit: one per couple per day ────────────────────────────────
 
   const today = new Date().toISOString().slice(0, 10) // e.g. "2026-06-19"
   const lockRef = db
@@ -53,7 +110,7 @@ export const sendGentleReminderCallable = functions.https.onCall(async (_data, c
     return { sent: false, reason: 'already_sent_today' }
   }
 
-  // ── 3. Collect partner FCM tokens ────────────────────────────────────────
+  // ── 4. Collect partner FCM tokens ────────────────────────────────────────
 
   const tokens: string[] = []
   const partnerDoc = await db.collection('users').doc(partnerId).get()
@@ -76,7 +133,7 @@ export const sendGentleReminderCallable = functions.https.onCall(async (_data, c
     }
   })
 
-  // ── 4. Write in-app notification record ──────────────────────────────────
+  // ── 5. Write in-app notification record ──────────────────────────────────
 
   await db
     .collection('users')
@@ -91,14 +148,14 @@ export const sendGentleReminderCallable = functions.https.onCall(async (_data, c
       createdAt: admin.firestore.FieldValue.serverTimestamp(),
     })
 
-  // ── 5. Claim the daily rate-limit lock ───────────────────────────────────
+  // ── 6. Claim the daily rate-limit lock ───────────────────────────────────
 
   await lockRef.set({
     sentBy: callerId,
     sentAt: admin.firestore.FieldValue.serverTimestamp(),
   })
 
-  // ── 6. Send FCM push ─────────────────────────────────────────────────────
+  // ── 7. Send FCM push ─────────────────────────────────────────────────────
 
   if (tokens.length > 0) {
     const sendResults = await Promise.allSettled(