perf: fix API CPU saturation at 400+ devices

Root cause: stale NATS JetStream consumers accumulated across API
restarts, causing 13+ consumers to fight over messages in a single
Python async event loop (100% CPU).

Fixes:
- Add performance indexes on devices(tenant_id, hostname),
  devices(tenant_id, status), key_access_log(tenant_id, created_at)
  — drops devices seq_scans from 402k to 6 per interval
- Remove redundant ORDER BY t.name from fleet summary SQL
  (tenant name sort is client-side, was forcing a cross-table sort)
- Bump NATS memory limit from 128MB to 256MB (was at 118/128)
- Increase dev poll interval from 60s to 120s for 400+ device fleet

The stream purge + restart brought API CPU from 100% to 0.3%.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/036_performance_indexes.py

@@ -0,0 +1,51 @@
+"""Add performance indexes for fleet dashboard and link discovery queries.
+
+Revision ID: 036
+Revises: 035
+Create Date: 2026-03-19
+
+At ~400 devices, sequential scans on the devices table accounted for 58M
+row reads. These indexes cover the hot query paths: fleet summary
+(tenant_id + hostname sort), dashboard status counts (tenant_id + status),
+and key_access_log time-range queries.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "036"
+down_revision = "035"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Fleet summary query: SELECT ... FROM devices JOIN tenants ORDER BY hostname
+    op.create_index(
+        "idx_devices_tenant_hostname",
+        "devices",
+        ["tenant_id", "hostname"],
+        if_not_exists=True,
+    )
+
+    # Dashboard status count queries
+    op.create_index(
+        "idx_devices_tenant_status",
+        "devices",
+        ["tenant_id", "status"],
+        if_not_exists=True,
+    )
+
+    # key_access_log: growing unbounded, queried by tenant + time range
+    op.create_index(
+        "idx_key_access_log_tenant_time",
+        "key_access_log",
+        [sa.text("tenant_id"), sa.text("created_at DESC")],
+        if_not_exists=True,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_key_access_log_tenant_time", table_name="key_access_log")
+    op.drop_index("idx_devices_tenant_status", table_name="devices")
+    op.drop_index("idx_devices_tenant_hostname", table_name="devices")

backend/app/routers/metrics.py

@@ -363,7 +363,7 @@ _FLEET_SUMMARY_SQL = """
         d.tenant_id, t.name AS tenant_name
     FROM devices d
     JOIN tenants t ON d.tenant_id = t.id
-    ORDER BY t.name, d.hostname
+    ORDER BY d.hostname
 """