feat(core/backend): pipeline continuity + threaded ffmpeg burn-in export + upload/save-timeline

Video pipeline (script→assets→storyboard→video→stitch):
- robust split_script_into_segments (4 non-empty scenes), scene-aware storyboard/video prompts
- link VideoSegment→ScriptSegment + storyboard-frame reference image (graceful text fallback)
- idempotent poll_video_segment (no double-charge on repeated polling)
- threaded export (no Celery worker needed) + poll-export endpoint
- run_export_job rewritten to filter_complex: per-clip trim, xfade transitions,
  subtitle burn-in (Pillow PNG overlay; this ffmpeg lacks libass), BGM mix
- upload-video-segment / upload-bgm / save-timeline endpoints
- serializers embed asset preview URLs (beat assets pagination); Pillow added to requirements

Also includes prior uncommitted backend work: account preferences/sessions,
billing trend, product/asset endpoints, accounts 0002 migration.

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

core/backend/apps/accounts/models.py

@@ -34,6 +34,44 @@ class Team(TimeStampedModel):
         return self.name
 
 
+def _default_notify() -> dict:
+    return {"n-export": True, "n-fail": True, "n-quota": True, "n-login": True}
+
+
+def _default_creation() -> dict:
+    return {"template": "pain", "duration": "60", "subtitle": "big-variety", "bgm": "kapian", "transition": "fade"}
+
+
+def _default_display() -> dict:
+    return {"appearance": "system", "language": "zh", "density": "standard"}
+
+
+class UserPreference(TimeStampedModel):
+    """用户设置:通知策略 / 两步验证 / 创作默认 / 显示偏好。服务端持久化(替代前端 localStorage)。"""
+
+    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name="preference")
+    notify = models.JSONField(default=_default_notify, blank=True)
+    two_factor_enabled = models.BooleanField(default=False)
+    creation_defaults = models.JSONField(default=_default_creation, blank=True)
+    display = models.JSONField(default=_default_display, blank=True)
+
+    def __str__(self) -> str:
+        return f"prefs/{self.user}"
+
+
+class LoginSession(TimeStampedModel):
+    """登录会话记录:每次登录写一条(设备 UA / IP / 时间),供设置页「在用设备」展示与下线。"""
+
+    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="login_sessions")
+    user_agent = models.CharField(max_length=400, blank=True)
+    ip_address = models.GenericIPAddressField(null=True, blank=True)
+    last_seen_at = models.DateTimeField(auto_now=True)
+    revoked_at = models.DateTimeField(null=True, blank=True)
+
+    class Meta:
+        ordering = ["-last_seen_at"]
+
+
 class TeamMember(TimeStampedModel):
     class Role(models.TextChoices):
         OWNER = "owner", "Owner"