Implement push notifications Phase 1 (FCM + project relay)

Backend (Rust):
- push_tokens migration, model (PushToken/PushPlatform), and db layer
  (upsert/list/delete/delete_many)
- PUT/DELETE /api/push_tokens endpoints wired into the router + OpenAPI
- "push" notification method: relay sender (reqwest) that forwards only the
  sanitized title/body and prunes dead tokens, plus settings with a default
  relay_url
- 164 tests pass, clippy clean

Relay (push_relay/, TypeScript Firebase Cloud Function):
- POST /v1/push (firebase-admin sendEach + per-token status mapping),
  GET /healthz, payload validation, per-IP Firestore rate limiting
- Jest tests + README documenting the manual project-owned setup

Frontend (Flutter):
- oott_api_push.dart (register/unregister), push_service.dart behind a
  PushService abstraction, and a per-device push toggle in settings
- firebase_core/firebase_messaging/flutter_local_notifications deps
- 145 tests pass, dart analyze clean

Dev shell:
- add nodejs_22 to the Nix dev shell so the relay tests/build run locally

Remaining (manual, project-owned): create the Firebase project, deploy the
relay and set the real default_relay_url, add native Firebase config, and test
on real devices.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/notifications/error.rs

@@ -1,8 +1,12 @@
 use std::{error, fmt};
 
+use crate::db::error::DbError;
+
 #[derive(Debug)]
 pub enum DeliveryError {
     ParsePushover(pushover::Error),
+    Http(reqwest::Error),
+    Db(DbError),
 }
 
 impl fmt::Display for DeliveryError {
@@ -11,6 +15,12 @@ impl fmt::Display for DeliveryError {
             DeliveryError::ParsePushover(..) => {
                 write!(f, "Error delivering notification via Pushover")
             }
+            DeliveryError::Http(..) => {
+                write!(f, "Error delivering notification via the push relay")
+            }
+            DeliveryError::Db(..) => {
+                write!(f, "Database error while delivering a push notification")
+            }
         }
     }
 }
@@ -19,6 +29,8 @@ impl error::Error for DeliveryError {
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match *self {
             DeliveryError::ParsePushover(ref e) => Some(e),
+            DeliveryError::Http(ref e) => Some(e),
+            DeliveryError::Db(ref e) => Some(e),
         }
     }
 }
@@ -28,3 +40,15 @@ impl From<pushover::Error> for DeliveryError {
         DeliveryError::ParsePushover(err)
     }
 }
+
+impl From<reqwest::Error> for DeliveryError {
+    fn from(err: reqwest::Error) -> DeliveryError {
+        DeliveryError::Http(err)
+    }
+}
+
+impl From<DbError> for DeliveryError {
+    fn from(err: DbError) -> DeliveryError {
+        DeliveryError::Db(err)
+    }
+}