Adding Manual connection point to allow undeclared fits

app/models/attachment_point.py

@@ -21,7 +21,10 @@ class AttachmentPoint(Base):
         ForeignKey("attachment_point_names.id", ondelete="RESTRICT"), nullable=False
     )
 
-    # Exactly one of the four below must be non-null (enforced in application layer)
+    # At most one of the four below is non-null (enforced in application layer).
+    # All four null means a "Manual" attachment point: no connection details,
+    # used to let a user assert a Fit by hand without the fit-matching algorithm
+    # (app.services.fit_matching) ever auto-matching it.
     iso_thread_id: Mapped[int | None] = mapped_column(
         ForeignKey("iso_threads.id", ondelete="RESTRICT"), nullable=True, default=None
     )