M0: backend skeleton — Quart factory, async DB, auth, ACL spine, migrations

Foundation & Identity backend for ThoughtSync:
- Quart app factory (create_app) with /api/health + SPA history-fallback
- async SQLAlchemy 2.0 + asyncpg engine/session (lazy; boots without a DB)
- native email+password auth via signed-cookie session (register/login/logout/me
  + login_required guard); bcrypt password hashing (72-byte safe)
- multi-user sharing-ACL spine (rule 47): users, groups, group_members, and a
  polymorphic shares table + visible_to_user() SQL predicate (owner OR direct
  share OR group share) that M1's notes will scope through
- Alembic async env (adapted from family pattern) + 0001 foundation migration
- DB-free unit tests (app/health/auth-guard, password roundtrip, ACL compile)

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

alembic/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}