feat(systems): the area vocabulary becomes a global table so a rule can point at one (#3027, milestone 307 step 1)

The eight standard area names already existed — as STANDARD_SYSTEMS, a tuple in
services/systems.py that milestone 297 seeds at inception. A constant cannot be
a foreign key, so nothing outside a project could reference an area: systems.
project_id is NOT NULL, and a rule that spans projects would have to chain
itself to one project's row. And because the list only ever applied on the
inception-seed path, three spellings of one area reached this instance anyway
(CI & runners / CI and Release / CI & release).

- canonical_systems: global, no user_id — a shared project inherits the
  vocabulary instead of re-earning it. Migration 0087 seeds the same eight.
- systems.canonical_id: nullable, SET NULL. Association only — no System is
  renamed and record_systems is untouched, so no record's tags move.
- canonical_slug folds &/and, case and punctuation, so spelling variants map
  mechanically and a real difference ("CI & runners") becomes a proposal a
  human confirms. propose_mappings reports; set_system_canonical is the only
  writer.
- seed_standard_systems now reads the catalog and maps as it mints, so a
  project born standard never needs a reconciliation pass.
- Catalog writes are admin-only; reads are open — a global list anyone can
  extend stops being shared.
- backup: carried by SLUG, not id (ids are per-install). Restore reuses the
  target's own rows and only creates entries an admin added on the source; an
  unknown slug restores unmapped rather than failing.

Rule 22: STANDARD_SYSTEMS is removed, not deprecated. Rule 115: nothing seeded
names an app, repo or house convention. Design in note 3026.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/scribe/models/system.py

@@ -24,6 +24,15 @@ class System(Base, TimestampMixin, SoftDeleteMixin):
         Integer, ForeignKey("projects.id", ondelete="CASCADE")
     )
     name: Mapped[str] = mapped_column(Text, default="", server_default="")
+    # The GLOBAL area this local System is an instance of (milestone 307).
+    # Nullable and SET NULL on purpose: the catalog is a convergence aid, not a
+    # gate — an unmapped System is fully usable, and retiring a canonical entry
+    # must never take a project's System with it. The local `name` is NEVER
+    # rewritten to match the canonical one; this column is the join key, and
+    # the name stays whatever the project calls the area.
+    canonical_id: Mapped[int | None] = mapped_column(
+        Integer, ForeignKey("canonical_systems.id", ondelete="SET NULL"), nullable=True
+    )
     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.
@@ -40,6 +49,7 @@ class System(Base, TimestampMixin, SoftDeleteMixin):
             "user_id": self.user_id,
             "project_id": self.project_id,
             "name": self.name,
+            "canonical_id": self.canonical_id,
             "description": self.description,
             "color": self.color,
             "status": self.status,