feat(backend): add pagination + user/member/audit DTOs (D-016)

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.

backend/src/mimic/schemas/pagination.py

@@ -0,0 +1,40 @@
+"""Generic pagination envelope (D-016).
+
+Frontend reads `{items, total, page, page_size}`. Sprint 2 uses this on
+`/users` and `/audit/log`; existing endpoints (`/engagements`) stay
+non-paginated for backwards-compatibility and will migrate together in a
+later opt-in (`?paginate=true` or `/api/v2/`).
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, ConfigDict, Field
+
+DEFAULT_PAGE_SIZE = 50
+MAX_PAGE_SIZE = 200
+
+
+class Page[T](BaseModel):
+    """Paginated response envelope."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    items: list[T]
+    total: int = Field(ge=0)
+    page: int = Field(ge=1)
+    page_size: int = Field(ge=1, le=MAX_PAGE_SIZE)
+
+
+class PageQuery(BaseModel):
+    """Parsed `?page=` / `?page_size=` query string (always normalised)."""
+
+    page: int = Field(default=1, ge=1)
+    page_size: int = Field(default=DEFAULT_PAGE_SIZE, ge=1, le=MAX_PAGE_SIZE)
+
+    @property
+    def offset(self) -> int:
+        return (self.page - 1) * self.page_size
+
+    @property
+    def limit(self) -> int:
+        return self.page_size