feat(store): миграция sessions/password + методы users/sessions/projects

Фаза 2, Task 1: добавлена таблица sessions и nullable password_hash у
users, sqlc-запросы и *Store-обёртки (CreateUser, GetUserByEmail,
CreateProjectForUser, GetProjectOwned, GetUserProject, CreateSession,
GetSessionUser, DeleteSession, RegisterUser в транзакции), интеграционные
тесты на testcontainers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/store/migrations/0003_auth.sql

@@ -0,0 +1,15 @@
+-- +goose Up
+ALTER TABLE users ADD COLUMN password_hash text;
+
+CREATE TABLE sessions (
+    id         uuid PRIMARY KEY,
+    user_id    uuid NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    token_hash text NOT NULL UNIQUE,
+    expires_at timestamptz NOT NULL,
+    created_at timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX sessions_token_hash_idx ON sessions (token_hash);
+
+-- +goose Down
+DROP TABLE sessions;
+ALTER TABLE users DROP COLUMN password_hash;