feat(backend): add Flask app factory, audit writer, flat CRUD + CLI (B0.7)

- Flask app factory wires SQLAlchemy / Migrate / Login / SocketIO and
  registers every blueprint. /healthz smoke endpoint included.
- Pydantic 2 DTOs (request/response) for engagement / host / TTP /
  scenario aggregates with from_attributes=True conversion.
- Flat CRUD blueprints under /api/v1/:
  * engagements (list / create / get / put / delete-as-archive)
  * hosts (engagement-scoped CRUD)
  * library/ttps (CRUD; promote requires the lead-only TTP_PROMOTE)
  * scenarios + steps (F3 invariant enforced: host.c2_type must match
    scenario.c2_type at compose time, 400 otherwise).
- @require_perm guards every endpoint per the F11 matrix.
- audit/ writer is hash-chained from v1 (SHA-256 of canonical record
  plus previous hash). The SQL-level write-only role enforcement ships
  in the deploy playbook (idempotent grants run at migration time).
- mimic-cli (click): user create (seeds RT operator/lead with group
  membership), db dump / db restore (manual pg_dump/pg_restore, R-O1).

No orchestrator, no WebSocket, no report generation — those land after
PR1/PR2/PR3.

backend/src/mimic/cli/__init__.py

@@ -0,0 +1,21 @@
+"""`mimic-cli` command-line interface (click)."""
+
+from __future__ import annotations
+
+import click
+
+from mimic.cli.db import db_group
+from mimic.cli.user import user_group
+
+
+@click.group()
+def cli() -> None:
+    """Mimic command-line interface."""
+
+
+cli.add_command(user_group, name="user")
+cli.add_command(db_group, name="db")
+
+
+if __name__ == "__main__":
+    cli()