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

src/thoughtsync/models/label.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Text, UniqueConstraint, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, Text, UniqueConstraint, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -34,3 +34,6 @@ class NoteLabel(Base):
     label_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("labels.id", ondelete="CASCADE"), primary_key=True
     )
+    # True when this attachment came from a #tag in the note body (kept in sync with
+    # the body); False = added manually via the label picker (independent of #tags).
+    via_tag: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())