Add origin_app to APActivity — apps only process their own activities

Each app's EventProcessor now filters by origin_app so apps don't steal
each other's pending activities. emit_activity() and publish_activity()
auto-detect the app name from Quart's current_app.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/federation.py

@@ -108,6 +108,9 @@ class APActivity(Base):
     processed_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True,
     )
+    origin_app: Mapped[str | None] = mapped_column(
+        String(64), nullable=True,
+    )
 
     # Relationships
     actor_profile = relationship("ActorProfile", back_populates="activities")