Adds the `Page[T]` envelope `{items, total, page, page_size}` documented in
D-016, the matching `PageQuery` for `?page=&page_size=` parsing (default 50,
max 200), and `parse_page_query()` helper for blueprints.
DTOs:
- `UserRead` / `UserCreate` / `UserUpdate` (sprint 2). `UserRead` never
exposes `local_password_hash`. `UserCreate` validates email via
pydantic-email-validator and pins password to 8..128 chars.
- `EngagementMemberRead` / `EngagementMemberCreate`. `role` is a free-form
string ≤ 40 chars (D-017), defaulting to `"member"`.
- `AuditLogEntry` for the upcoming audit viewer.
