m4.5: inline #tags — hashtags in a note become labels (two-way sync)

Fast, cross-device labelling: type #groceries in a note and it becomes
the "groceries" label. The body is the source of truth for tag-labels;
manual picker labels stay independent (rule 28 — additive).

- note_labels.via_tag (migration 0013) marks tag-sourced attachments.
- parse_tags(): #tag at start-of-body or after whitespace, needs a
  letter (so #2024, URL #frags, mid#word are ignored). unit-tested.
- _reconcile_tags() on create + body-update: attach labels for current
  #tags (find-or-create, case-insensitive), detach tag-labels whose tag
  was removed; never touches manual (via_tag=false) rows.
- label picker (set_note_labels + editor onLabelsChange) now preserves
  tag-labels on save, so a picker action can't strip a label the #tag
  still mandates.
- serialize via_tag; card/editor chips render tag-labels as "#name",
  and the editor hides the × on them (remove by editing the tag text).
- LabelPicker builds manual NoteLabels (via_tag:false).

Fourth and final item of M4.5.

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

alembic/versions/0013_note_label_via_tag.py

@@ -0,0 +1,26 @@
+"""note_labels.via_tag
+
+Revision ID: 0013
+Revises: 0012
+Create Date: 2026-07-22
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0013"
+down_revision = "0012"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Marks a note↔label attachment that came from a body #tag (vs. the picker).
+    # Existing attachments default to False = manual, which is correct.
+    op.add_column(
+        "note_labels",
+        sa.Column("via_tag", sa.Boolean(), nullable=False, server_default=sa.false()),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("note_labels", "via_tag")