fix(#142): index device_status_log + de-dupe its CREATE TABLE

The dashboard uptime query (WHERE device_id=? AND timestamp>?) and the
per-device retention prune (WHERE device_id=? AND timestamp<?) were both full
table scans. At 1M+ rows (the outage report) this was the dashboard-degradation
cause that persisted even after the reconnect storm stopped.

- schema.sql: add idx_device_status_log_device_ts(device_id, timestamp); both
  queries now SEARCH ... USING INDEX instead of SCAN (verified via EXPLAIN).
- database.js: same index as a migration for existing DBs (idempotent).
- schema.sql defined device_status_log twice; drop the duplicate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/db/database.js

@@ -225,6 +225,11 @@ const migrations = [
   "ALTER TABLE devices ADD COLUMN ota_target_version TEXT",
   "ALTER TABLE devices ADD COLUMN ota_attempts INTEGER DEFAULT 0",
   "ALTER TABLE devices ADD COLUMN ota_updated_at INTEGER",
+  // #142: index device_status_log for the per-device + time-window access pattern.
+  // schema.sql creates this on fresh installs; this migration covers existing DBs.
+  // Both the dashboard uptime query and the retention prune were full scans — the
+  // dashboard-degradation cause once the table reached 1M+ rows.
+  "CREATE INDEX IF NOT EXISTS idx_device_status_log_device_ts ON device_status_log(device_id, timestamp)",
 ];
 // Apply each ALTER idempotently. A "duplicate column name" / "already exists"
 // error means the column is already present (expected on a migrated DB) - benign.

server/db/schema.sql

@@ -463,6 +463,11 @@ CREATE TABLE IF NOT EXISTS device_status_log (
     status          TEXT NOT NULL,
     timestamp       INTEGER NOT NULL DEFAULT (strftime('%s','now'))
 );
+-- #142: index the per-device + time-window access pattern. Both the dashboard
+-- uptime query (WHERE device_id=? AND timestamp>?) and the retention prune
+-- (WHERE device_id=? AND timestamp<?) were full table scans; at 1M+ rows that
+-- was the dashboard-degradation cause in the outage report.
+CREATE INDEX IF NOT EXISTS idx_device_status_log_device_ts ON device_status_log(device_id, timestamp);
 
 -- ===================== DEVICE FINGERPRINTS =====================
 
@@ -484,13 +489,6 @@ CREATE TABLE IF NOT EXISTS alert_configs (
     created_at      INTEGER NOT NULL DEFAULT (strftime('%s','now'))
 );
 
-CREATE TABLE IF NOT EXISTS device_status_log (
-    id              INTEGER PRIMARY KEY AUTOINCREMENT,
-    device_id       TEXT NOT NULL,
-    status          TEXT NOT NULL,
-    timestamp       INTEGER NOT NULL DEFAULT (strftime('%s','now'))
-);
-
 -- ===================== PLAYER DEBUG LOGS =====================
 -- Smart TVs (Tizen, WebOS, Fire TV, etc.) have no accessible devtools. The
 -- player captures errors into window.__debugLog client-side and POSTs them