feat: Google OAuth login + server sessions (user_key u:<sub>)

backend/internal/store/migrations/0005_auth.sql

@@ -0,0 +1,19 @@
+-- FlowSight schema v5: Google-authenticated users + server sessions.
+-- user_key for a login user is `u:<google_sub>`; all user-scoped tables
+-- (watchlists, routines, alerts, destinations, reports) work unchanged.
+CREATE TABLE IF NOT EXISTS users(
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  google_sub TEXT NOT NULL UNIQUE,
+  email TEXT NOT NULL,
+  name TEXT NOT NULL DEFAULT '',
+  avatar_url TEXT NOT NULL DEFAULT '',
+  created_at TEXT NOT NULL DEFAULT ''
+);
+CREATE TABLE IF NOT EXISTS sessions(
+  token TEXT PRIMARY KEY,
+  user_key TEXT NOT NULL,
+  user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  expires_at TEXT NOT NULL,
+  created_at TEXT NOT NULL DEFAULT ''
+);
+CREATE INDEX IF NOT EXISTS idx_sessions_user ON sessions(user_id);