labels: per-label color

Give labels a color (migration 0011 adds labels.color, server_default
'default' so existing labels keep the neutral chip). The PATCH endpoint now
updates name and/or color; note serialization carries each label's color.
Frontend: a swatch picker per label in the Edit-labels modal, colored chips
on cards + in the editor (LABEL_CHIP_CLASSES), and a color dot on each
sidebar label. Reuses the note color vocabulary. (Graph node coloring rides
this in the graph-liveliness task.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

alembic/versions/0011_label_color.py

@@ -0,0 +1,22 @@
+"""labels.color
+
+Revision ID: 0011
+Revises: 0010
+Create Date: 2026-07-20
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0011"
+down_revision = "0010"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # server_default backfills existing labels to the neutral "default" chip.
+    op.add_column("labels", sa.Column("color", sa.Text(), nullable=False, server_default="default"))
+
+
+def downgrade() -> None:
+    op.drop_column("labels", "color")