feat(issues): S1 schema — issue task_kind, System entity, associations

First slice of the Issues + Systems feature (spec #825, plan #819 T2).

Schema (migration 0065):
- task_kind CHECK expands work|plan -> work|plan|issue (same-change, rule 36)
- notes.arose_from_id: optional self-FK for issue->originating-task provenance
  (distinct from parent_id sub-task hierarchy)
- systems: per-project, self-describing (name + description) subsystem/area
- record_systems: M2M join linking any note/task/issue to systems (mutable)

Models: System + RecordSystem; note.py gains arose_from_id (+ index, to_dict).
Service services/systems.py: CRUD, archive, soft-delete, set/list associations,
records-for-system, open-issue count — all gated via services/access.py project
permissions (rule 78, no bare-owner filters). Unit tests lock the ACL gating;
the migration is exercised by CI's integration lane (alembic upgrade head).

is_task stays a derived property (status is not None) — unchanged. T1 (typing-
axis rationalization) intentionally NOT bundled; this only adds the enum value.

Refs plan 825 (S1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/models/system.py

@@ -0,0 +1,73 @@
+from sqlalchemy import ForeignKey, Index, Integer, Text, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from scribe.models import Base
+from scribe.models.base import CreatedAtMixin, TimestampMixin, SoftDeleteMixin
+
+
+class System(Base, TimestampMixin, SoftDeleteMixin):
+    """A per-project, reusable, self-describing subsystem/area.
+
+    Any record (note, task, or issue) can be associated with one or more
+    systems via record_systems, so research notes, build-tasks, and corrective
+    work line up under the same area — and recurring problem-areas become
+    filterable. Self-describing (name + description) because a bare name is
+    never enough to convey what a system is or how it's used.
+    """
+    __tablename__ = "systems"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE")
+    )
+    project_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("projects.id", ondelete="CASCADE")
+    )
+    name: Mapped[str] = mapped_column(Text, default="", server_default="")
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    color: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # active | archived — systems accumulate; archive rather than delete.
+    status: Mapped[str] = mapped_column(Text, default="active", server_default="active")
+    order_index: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
+
+    __table_args__ = (
+        Index("ix_systems_project_id", "project_id"),
+    )
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "user_id": self.user_id,
+            "project_id": self.project_id,
+            "name": self.name,
+            "description": self.description,
+            "color": self.color,
+            "status": self.status,
+            "order_index": self.order_index,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+
+class RecordSystem(Base, CreatedAtMixin):
+    """M2M association: a record (notes.id — any note/task/issue) <-> a System.
+
+    Mutable over time; uniqueness keeps a record from linking the same system
+    twice. Cascades on both sides, so deleting a record or a system clears its
+    associations.
+    """
+    __tablename__ = "record_systems"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    note_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("notes.id", ondelete="CASCADE")
+    )
+    system_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("systems.id", ondelete="CASCADE")
+    )
+
+    __table_args__ = (
+        UniqueConstraint("note_id", "system_id", name="uq_record_systems_note_system"),
+        Index("ix_record_systems_note_id", "note_id"),
+        Index("ix_record_systems_system_id", "system_id"),
+    )