feat(backend): add §8 data model + Alembic baseline (B0.2, B0.3)

- SQLAlchemy 2 typed mapped classes for every spec §8 aggregate:
  engagement, c2_credential, host, user, group, group_permission,
  user_group, engagement_member, ttp, ttp_version, scenario,
  scenario_step, run, run_step, run_step_cleanup, detection, evidence,
  report, soc_session, audit_log.
- Shared mixins: UuidPkMixin (PG_UUID(as_uuid=True)) + TimestampsMixin.
- StrEnum types covering every spec enum (C2Type, PayloadType, UserType,
  EngagementStatus, HostStatus, TtpSource, RunStatus, RunStepStatus,
  CleanupStatus, DetectionLevel, DetectionSource, EvidenceStatus).
- Alembic baseline migration 202605210001_initial_schema: creates every
  table, enum, index, and idempotent grants for the audit_log
  write-only Postgres role (mimic_audit_writer).
- Audit log carries prev_hash / row_hash from v1 (D-009).
- ttp_version table coexists with run.snapshot_json (D-008,
  overrides H32).

backend/src/mimic/db/models/report.py

@@ -0,0 +1,48 @@
+"""Mission report (PDF / JSON / Markdown bundle)."""
+
+from __future__ import annotations
+
+from datetime import datetime
+from typing import TYPE_CHECKING
+from uuid import UUID
+
+from sqlalchemy import (
+    JSON,
+    DateTime,
+    ForeignKey,
+    Integer,
+    String,
+)
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from mimic.db.base import Base, TimestampsMixin, UuidPkMixin
+
+if TYPE_CHECKING:
+    from mimic.db.models.engagement import Engagement
+
+
+class Report(UuidPkMixin, TimestampsMixin, Base):
+    __tablename__ = "report"
+
+    engagement_id: Mapped[UUID] = mapped_column(
+        ForeignKey("engagement.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    version: Mapped[int] = mapped_column(Integer, default=1, nullable=False)
+
+    content_json: Mapped[dict] = mapped_column(JSON, nullable=False, default=dict)
+    content_sha256: Mapped[str] = mapped_column(String(64), nullable=False)
+    # SHA-256 of canonical JSON. Identical hash referenced in PDF footer, JSON
+    # export and Markdown export (spec H19 / F9 / F14).
+
+    pdf_path: Mapped[str | None] = mapped_column(String(512))
+    md_path: Mapped[str | None] = mapped_column(String(512))
+
+    generated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )
+    generated_by_id: Mapped[UUID | None] = mapped_column(
+        ForeignKey("user.id", ondelete="SET NULL")
+    )
+
+    engagement: Mapped[Engagement] = relationship()