feat: add batch_id/target_label to sessions and batch launch endpoint

- Add batch_id (UUID, nullable, indexed) and target_label (String 255,
  nullable) columns to the Session model
- Manual Alembic migration 6e8128ef2aa8 applies both columns
- POST /sessions/batch creates one session per target for maintenance
  flows; rejects empty targets (422) and non-maintenance trees (400)
- SessionResponse schema exposes batch_id and target_label
- 3 new integration tests, all 540 tests pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/session.py

@@ -65,3 +65,11 @@ class Session(Base):
     user: Mapped["User"] = relationship("User", back_populates="sessions")
     attachments: Mapped[list["Attachment"]] = relationship("Attachment", back_populates="session")
     shares: Mapped[list["SessionShare"]] = relationship("SessionShare", back_populates="session", cascade="all, delete-orphan")
+
+    # Batch tracking (maintenance flows)
+    batch_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True), nullable=True, index=True
+    )
+    target_label: Mapped[Optional[str]] = mapped_column(
+        String(255), nullable=True
+    )