Add account self-delete and subscription lifecycle sync (#34)

Soft-delete DELETE /api/user/ for authenticated users (hide conversations,
blacklist tokens, block staff self-delete). Sync Stripe portal cancel/change
via subscription.updated/deleted webhooks and expose cancel_at_period_end for
Account UI (chat_web_app#75 companion).

llm_be/finance/services/plans.py

@@ -221,17 +221,66 @@ def assign_founders_from_stripe(
     *,
     stripe_subscription_id: str = "",
 ) -> UserSubscription:
+    """Backward-compatible helper; prefer ``assign_plan_from_stripe``."""
+    return assign_plan_from_stripe(
+        user,
+        plan_slug=SubscriptionPlan.Slug.FOUNDERS,
+        stripe_subscription_id=stripe_subscription_id,
+    )
+
+
+def assign_plan_from_stripe(
+    user,
+    *,
+    plan_slug: str | None = None,
+    stripe_subscription_id: str = "",
+    status: str = UserSubscription.Status.ACTIVE,
+    cancel_at_period_end: bool | None = None,
+    current_period_end=None,
+    keep_existing_plan_if_unknown: bool = False,
+) -> UserSubscription:
+    """Assign a catalog plan from a Stripe Checkout / subscription event."""
     seed_subscription_plans(update_existing=False)
-    plan = get_plan(SubscriptionPlan.Slug.FOUNDERS)
+    slug = (plan_slug or "").strip().lower()
+    plan = get_plan(slug) if slug else None
+    if plan is None and keep_existing_plan_if_unknown:
+        existing = UserSubscription.objects.filter(user=user).select_related("plan").first()
+        if existing and existing.plan_id:
+            plan = existing.plan
+    if plan is None:
+        if slug:
+            logger.warning(
+                "Unknown plan_slug=%s; falling back to Founders for user=%s",
+                slug,
+                getattr(user, "pk", None),
+            )
+        plan = get_plan(SubscriptionPlan.Slug.FOUNDERS)
     if plan is None:
         raise RuntimeError("Founders plan missing from catalog")
-    return assign_plan(
+    sub = assign_plan(
         user,
         plan=plan,
         source=UserSubscription.Source.STRIPE,
-        status=UserSubscription.Status.ACTIVE,
+        status=status,
         stripe_subscription_id=stripe_subscription_id or "",
     )
+    update_fields: list[str] = []
+    if cancel_at_period_end is not None:
+        sub.cancel_at_period_end = bool(cancel_at_period_end)
+        update_fields.append("cancel_at_period_end")
+    if current_period_end is not None:
+        sub.current_period_end = current_period_end
+        update_fields.append("current_period_end")
+    if update_fields:
+        sub.save(update_fields=update_fields)
+    return sub
+
+
+def resolve_plan_from_stripe_price(price_id: str | None) -> SubscriptionPlan | None:
+    """Map a Stripe Price id to a local SubscriptionPlan when configured."""
+    if not price_id:
+        return None
+    return SubscriptionPlan.objects.filter(stripe_price_id=price_id).first()
 
 
 def plan_to_dict(plan: SubscriptionPlan | None) -> dict[str, Any] | None: