feat(backend): replace auth stub with real session, create users and sessions tables

backend/migrations/20260523200650_create_users.sql

@@ -0,0 +1,6 @@
+CREATE TABLE users (
+    id              BIGSERIAL       PRIMARY KEY,
+    username        TEXT            NOT NULL,
+    password_hash   TEXT            NOT NULL,
+    created_at      TIMESTAMPTZ     NOT NULL DEFAULT NOW()
+);

backend/migrations/20260523200652_create_sessions.sql

@@ -0,0 +1,6 @@
+CREATE TABLE sessions (
+    id              TEXT            PRIMARY KEY,
+    user_id         BIGINT          NOT NULL REFERENCES users(id),
+    expires_at      TIMESTAMPTZ     NOT NULL,
+    created_at      TIMESTAMPTZ     NOT NULL DEFAULT NOW()
+);