0.2.33: exited positions (secondary-sale edge case)

A member who sold/transferred their stake showed a phantom -100% loss
($0 ending balance, no distribution through the fund). Now:
- entity_access.exited_on (migration e1f2a3b4c5d6), set/cleared from the
  Partners tab (writer-only, inline date picker, audited)
- LP portal card shows a quiet "Exited <date>" badge, keeps documents,
  hides balance/gain
- portfolio summary excludes exited positions ("Excludes N exited")
- fund committed totals (rollup + Partners tab) skip exited members so
  seller + buyer are not double-counted

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/ten31portal/models.py

@@ -175,6 +175,10 @@ class EntityAccess(SQLModel, table=True):
     id: int | None = Field(default=None, primary_key=True)
     user_id: int = Field(foreign_key="users.id")
     entity_id: int = Field(foreign_key="entities.id", index=True)
+    # Set when this member sold/transferred their stake (e.g. a secondary sale): the fund's
+    # books show a $0 balance with no distribution, which is NOT a loss. An exited position
+    # shows a badge instead of gain/loss, keeps its documents, and drops out of totals.
+    exited_on: date | None = Field(default=None)
     created_at: datetime = Field(default_factory=datetime.utcnow)