feat(l1): add L1 columns + extend account_role CHECK constraint

Adds users.can_cover_l1, accounts.l1_seats_purchased, subscriptions.l1_seat_limit,
audit_logs.acting_as. Rotates the users.account_role CHECK constraint to include
'l1_tech' (was: 'owner', 'admin', 'engineer', 'viewer').

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/models/audit_log.py

@@ -35,6 +35,7 @@ class AuditLog(Base):
     )
     details: Mapped[Optional[dict]] = mapped_column(JSONB, nullable=True)
     ip_address: Mapped[Optional[str]] = mapped_column(String(45), nullable=True)
+    acting_as: Mapped[Optional[str]] = mapped_column(String(30), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc)