feat: add SQLite migration and DB init with WAL mode

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

LORAWEB_NEU/database/migrations/001_initial.sql

@@ -0,0 +1,33 @@
+-- database/migrations/001_initial.sql
+
+CREATE TABLE IF NOT EXISTS topics (
+    id TEXT PRIMARY KEY,
+    topic TEXT UNIQUE NOT NULL,
+    approved INTEGER NOT NULL DEFAULT 0,
+    last_seen TEXT,
+    alarm_status TEXT NOT NULL DEFAULT 'unknown',
+    alarm_threshold_hours REAL NOT NULL DEFAULT 0.0,
+    created_at TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS sensors (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    location TEXT NOT NULL DEFAULT '',
+    sensor_type TEXT NOT NULL DEFAULT '',
+    active INTEGER NOT NULL DEFAULT 1,
+    topic_id TEXT REFERENCES topics(id),
+    display_config TEXT NOT NULL DEFAULT '{}',
+    created_at TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS users (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    username TEXT UNIQUE NOT NULL,
+    password_hash TEXT NOT NULL,
+    role TEXT NOT NULL DEFAULT 'user',
+    totp_secret TEXT,
+    totp_enabled INTEGER NOT NULL DEFAULT 0,
+    created_at TEXT NOT NULL
+);