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/api/engagements.py

@@ -0,0 +1,73 @@
+"""Engagement CRUD endpoints (flat, sprint 0)."""
+
+from __future__ import annotations
+
+from flask import Blueprint, abort, jsonify
+from sqlalchemy import select
+
+from mimic.api._helpers import jsonify_model, parse_body, parse_uuid
+from mimic.db.models import Engagement
+from mimic.db.types import EngagementStatus
+from mimic.extensions import db
+from mimic.rbac import Permission, require_perm
+from mimic.schemas import EngagementCreate, EngagementRead, EngagementUpdate
+
+bp = Blueprint("engagements", __name__)
+
+
+@bp.get("")
+@require_perm(Permission.ENGAGEMENT_READ)
+def list_engagements():
+    stmt = select(Engagement).order_by(Engagement.created_at.desc())
+    rows = db.session.execute(stmt).scalars().all()
+    return jsonify([EngagementRead.model_validate(row).model_dump(mode="json") for row in rows])
+
+
+@bp.post("")
+@require_perm(Permission.ENGAGEMENT_CREATE)
+def create_engagement():
+    payload = parse_body(EngagementCreate)
+    engagement = Engagement(
+        client_name=payload.client_name,
+        description=payload.description,
+        c2_type=payload.c2_type,
+        start_date=payload.start_date,
+        end_date=payload.end_date,
+        status=EngagementStatus.DRAFT,
+    )
+    db.session.add(engagement)
+    db.session.commit()
+    return jsonify_model(EngagementRead.model_validate(engagement), status=201)
+
+
+@bp.get("/<eid>")
+@require_perm(Permission.ENGAGEMENT_READ)
+def get_engagement(eid: str):
+    engagement = db.session.get(Engagement, parse_uuid(eid))
+    if engagement is None:
+        abort(404)
+    return jsonify_model(EngagementRead.model_validate(engagement))
+
+
+@bp.put("/<eid>")
+@require_perm(Permission.ENGAGEMENT_UPDATE)
+def update_engagement(eid: str):
+    engagement = db.session.get(Engagement, parse_uuid(eid))
+    if engagement is None:
+        abort(404)
+    payload = parse_body(EngagementUpdate)
+    for field, value in payload.model_dump(exclude_unset=True).items():
+        setattr(engagement, field, value)
+    db.session.commit()
+    return jsonify_model(EngagementRead.model_validate(engagement))
+
+
+@bp.delete("/<eid>")
+@require_perm(Permission.ENGAGEMENT_DELETE)
+def delete_engagement(eid: str):
+    engagement = db.session.get(Engagement, parse_uuid(eid))
+    if engagement is None:
+        abort(404)
+    engagement.status = EngagementStatus.ARCHIVED
+    db.session.commit()
+    return "", 204