feat: timer intervallo misura su ricetta + auto-logout per inattività

- Recipe: nuovo campo measurement_interval_minutes (migration 003) con UI nel recipe editor
- Auto-logout: impostazione di sistema configurabile da admin/settings, timer inattività JS
  con warning modale 60s prima del logout, tracking eventi utente throttled
- Navbar: aggiunto link "Impostazioni" per admin (desktop + mobile)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,24 @@
+"""add measurement_interval_minutes to recipes
+
+Revision ID: 003_measurement_interval
+Revises: 002_add_stations
+Create Date: 2026-05-23
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '003_measurement_interval'
+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('recipes', sa.Column('measurement_interval_minutes', sa.SmallInteger(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('recipes', 'measurement_interval_minutes')