feat(forge): per-user forge connections — keyring, host-keyed resolution, project pin (#2778)

A forge token is a user's credential, not an instance's. The single
admin-settings config is replaced by per-user keyring rows (one per forge
host), and every server-side forge read runs on the PROJECT OWNER's keyring:

- forge_connections table + projects.forge_connection_id pin (migration 0078,
  which also carries the existing admin config into the first admin's row and
  deletes the old setting keys — no legacy dual-read)
- get_forge() replaced by get_forges(owner_id, project_id) -> ForgeSelector;
  resolve(repo) picks the connection whose host serves the repo. A pinned
  project uses ONLY its pinned connection; a stale pin (ownership moved) is
  ignored, never honored across users
- env FORGE_* config survives as an implicit entry for admin owners only;
  a stored row for the same host beats it
- consumers threaded: pull-time freshness (owner of the note), coverage
  (owner of the project), coverage routes' configured flag
- routes: /api/settings/forge-connections CRUD + per-connection test
  (own-rows only, tokens never returned); /api/admin/forge shrinks to
  /api/admin/forge-webhook (secret only); PUT /api/projects/<id>/forge pins,
  owner-or-admin asking, owner's connections only
- UI: Git Forges card moves to Settings -> Integrations as a connection
  list; webhook secret stays in the admin Config tab; owner-only forge
  select on the project coverage card
- backups exclude forge_connections (credentials, api_keys precedent) and
  the pin, so restores fall back to keyring resolution

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/models/project.py

@@ -27,6 +27,15 @@ class Project(Base, TimestampMixin, SoftDeleteMixin):
     design_system_id: Mapped[int | None] = mapped_column(
         BigInteger, ForeignKey("design_systems.id", ondelete="SET NULL"), nullable=True
     )
+    # The per-project forge pin (#2778). NULL is the ordinary state: forge
+    # reads resolve against the owner's keyring by repo host. When set, the
+    # project's forge reads use ONLY this connection — an explicit, auditable
+    # choice, constrained by the service layer to a connection the project
+    # OWNER holds (never a collaborator's token).
+    forge_connection_id: Mapped[int | None] = mapped_column(
+        BigInteger, ForeignKey("forge_connections.id", ondelete="SET NULL"),
+        nullable=True,
+    )
 
     def to_dict(self) -> dict:
         return {
@@ -38,6 +47,7 @@ class Project(Base, TimestampMixin, SoftDeleteMixin):
             "status": self.status,
             "color": self.color,
             "design_system_id": self.design_system_id,
+            "forge_connection_id": self.forge_connection_id,
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),
         }