feat(measure): memorizza tempo di inserimento misura (input_duration_ms)

Colonna nullable su measurements + migration 003, campo opzionale negli
schemi API (create singola e batch), passthrough nel proxy Flask e timer
lato client in task_execute: parte all'attivazione del subtask, si
resetta su auto-advance e navigazione manuale.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/backend/migrations/versions/003_add_input_duration_ms.py

@@ -0,0 +1,27 @@
+"""add input_duration_ms to measurements
+
+Revision ID: 003_input_duration
+Revises: 002_add_stations
+Create Date: 2026-07-28
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '003_input_duration'
+down_revision: Union[str, None] = '002_add_stations'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'measurements',
+        sa.Column('input_duration_ms', sa.Integer, nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('measurements', 'input_duration_ms')