Revert "Give phase 2 somewhere to record a tag's instance identity"

This reverts 9aea271.

The column bought one thing, that phase 2 would need no migration, and
cost something worse: it commits the schema to a shape nobody has run a
decoder against. Width, and above all the choice not to make it unique,
were read off a design document rather than off working code, and
goeland86's codec in #880 computes an effective_instance_uuid that may
not mean the same thing. Getting that wrong leaves us altering a
populated column on four databases, which is the exact operation the
"settle the shape before release" rule exists to avoid.

That rule earns its keep for target_type, target_value and filament_id,
where retrofitting a discriminator or a nullability change is painful.
Appending one nullable column later is not painful, so phase 2 can add
it when there is something to write into it.

migrations/versions/2026_08_13_1000-fe4970567bb3_tags.py

@@ -39,7 +39,6 @@ def upgrade() -> None:
         sa.Column("filament_id", sa.Integer(), nullable=True),
         sa.Column("target_value", sa.String(length=64), nullable=True),
         sa.Column("format", sa.String(length=32), nullable=True),
-        sa.Column("instance_uuid", sa.String(length=64), nullable=True),
         sa.Column("added", sa.DateTime(), nullable=False),
         sa.ForeignKeyConstraint(
             ["spool_id"],
@@ -60,14 +59,10 @@ def upgrade() -> None:
     # The whole point of the table: one physical tag means exactly one thing, enforced by
     # the database rather than by whichever client happened to write last.
     op.create_index(op.f("ix_tag_uid"), "tag", ["uid"], unique=True)
-    # Not unique: see models.Tag. A payload copied onto a second sticker can carry one
-    # instance identity on two rows, both pointing at the same spool.
-    op.create_index(op.f("ix_tag_instance_uuid"), "tag", ["instance_uuid"], unique=False)
 
 
 def downgrade() -> None:
     """Perform the downgrade."""
-    op.drop_index(op.f("ix_tag_instance_uuid"), table_name="tag")
     op.drop_index(op.f("ix_tag_uid"), table_name="tag")
     op.drop_index(op.f("ix_tag_filament_id"), table_name="tag")
     op.drop_index(op.f("ix_tag_spool_id"), table_name="tag")

spoolman/database/models.py

@@ -197,15 +197,4 @@ class Tag(Base):
     # Free-ish string (openprinttag, ntag, bambu, ...) rather than an enum: informational
     # in phase 1, and new tag types appear faster than migrations should.
     format: Mapped[str | None] = mapped_column(String(32))
-    # Where phase 2 records a tag's decoded instance identity, so that rescanning a tag
-    # updates the spool it already created instead of making a second one. Nothing writes
-    # it yet -- it is here for the same reason the other unused columns are, that the
-    # table is the expensive half to change once it is populated on four databases, while
-    # the endpoint that fills it is additive and free.
-    #
-    # Indexed for that lookup and deliberately NOT unique. `uid` already enforces that one
-    # physical tag identifies one thing, and copying a tag's payload onto a second sticker
-    # (the case in the class docstring) can legitimately put one instance identity on two
-    # rows that point at the same spool -- which a unique constraint would reject.
-    instance_uuid: Mapped[str | None] = mapped_column(String(64), index=True)
     added: Mapped[datetime] = mapped_column()