feat: Phase 0 mobile app scaffold — Expo 55, SQLite, Feed/Import/Settings screens

mobile/db/index.ts

@@ -0,0 +1,26 @@
+import type { SQLiteDatabase } from 'expo-sqlite';
+
+export async function migrateDb(db: SQLiteDatabase): Promise<void> {
+  await db.execAsync('PRAGMA journal_mode = WAL;');
+  await db.execAsync(`
+    CREATE TABLE IF NOT EXISTS activities (
+      id              TEXT PRIMARY KEY,
+      source_hash     TEXT NOT NULL,
+      detail_json     TEXT NOT NULL,
+      timeseries_json TEXT,
+      geojson         TEXT,
+      original_path   TEXT,
+      synced_at       INTEGER,
+      origin          TEXT NOT NULL CHECK(origin IN ('local', 'remote')),
+      created_at      INTEGER NOT NULL DEFAULT (unixepoch())
+    );
+
+    CREATE INDEX IF NOT EXISTS idx_activities_created_at
+      ON activities(created_at DESC);
+
+    CREATE TABLE IF NOT EXISTS settings (
+      key   TEXT PRIMARY KEY,
+      value TEXT NOT NULL
+    );
+  `);
+}