fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/024_contact_email_and_offline_rule.py

@@ -19,7 +19,8 @@ def upgrade() -> None:
     op.add_column("tenants", sa.Column("contact_email", sa.String(255), nullable=True))
 
     # 2. Seed device_offline default alert rule for all existing tenants
-    conn.execute(sa.text("""
+    conn.execute(
+        sa.text("""
         INSERT INTO alert_rules (id, tenant_id, name, metric, operator, threshold, duration_polls, severity, enabled, is_default)
         SELECT gen_random_uuid(), t.id, 'Device Offline', 'device_offline', 'eq', 1, 1, 'critical', TRUE, TRUE
         FROM tenants t
@@ -28,14 +29,17 @@ def upgrade() -> None:
             SELECT 1 FROM alert_rules ar
             WHERE ar.tenant_id = t.id AND ar.metric = 'device_offline' AND ar.is_default = TRUE
           )
-    """))
+    """)
+    )
 
 
 def downgrade() -> None:
     conn = op.get_bind()
 
-    conn.execute(sa.text("""
+    conn.execute(
+        sa.text("""
         DELETE FROM alert_rules WHERE metric = 'device_offline' AND is_default = TRUE
-    """))
+    """)
+    )
 
     op.drop_column("tenants", "contact_email")