docs: push notification setup in README, fix VAPID_SUBJECT example

- .env.example: VAPID_SUBJECT without mailto: prefix (library adds it)
- README: VAPID env vars in table, push notification setup section
- README: add dm/ to project structure, keygen instructions
- README: iOS tap-gesture requirement and Android battery optimization notes

README.md

@@ -96,6 +96,7 @@ dumpsterChat/
 │   ├── invite/           # Server invite links
 │   ├── bot/              # Bot framework, auth, commands
 │   ├── webhook/          # Incoming webhooks
+│   ├── dm/               # Direct messages (conversations)
 │   ├── push/             # Push notification sender (VAPID)
 │   ├── permissions/      # Permission bitflags and checker
 │   ├── moderation/       # Mute, ban, kick, slowmode, audit log
@@ -133,6 +134,21 @@ See `.env.example` for the full list. Key variables:
 | `LIVEKIT_*` | Voice/video server | No |
 | `GIPHY_API_KEY` | Giphy API key for GIF search | No |
 | `MINIO_*` | File upload storage | No |
+| `VAPID_PUBLIC_KEY` | Web push public key | No (generate with `go run ./cmd/keygen`) |
+| `VAPID_PRIVATE_KEY` | Web push private key | No |
+| `VAPID_SUBJECT` | Contact for VAPID JWT (e.g. `admin@your.domain`) | No |
+
+### Push Notifications
+
+Web push uses VAPID keys. Generate a keypair:
+
+```bash
+go run ./cmd/keygen
+```
+
+Copy the output into your `.env`. Set `VAPID_SUBJECT` to your email address **without** the `mailto:` prefix (the library adds it automatically).
+
+Users are prompted to enable notifications on login. On iOS, the prompt requires a tap gesture, so a banner appears instead of a silent auto-subscribe. On Android, notifications deliver through FCM when Chrome is running (even in background). Ensure Chrome battery optimization is set to Unrestricted for reliable delivery when the PWA is closed.
 
 ## Wiki