feat(billing): reconcile plan taxonomy and add Stripe sync script

The marketing surface (PricingPage, Stripe products) was wired for
"Starter / Pro / Enterprise" while the backend was on "free / pro / team",
leaving plan_billing unseeded and BillingPlan accepting a literal that
violated the FK to plan_limits.

This change:

- Migration 4ce3e594cb87: defensive UPDATE of any subscriptions on
  plan='team' to 'enterprise' (dev has zero), renames the plan_limits
  row team -> enterprise, inserts a starter row with caps interpolated
  between free and pro (max_trees=10, sessions=75, ai=15/mo).
- Renames the plan tier across schemas (invite_code, billing, admin,
  subscription comment), is_paid/has_pro_entitlement checks in the
  Subscription model, admin/admin_dashboard plan validators, and the
  frontend useSubscription isPaidPlan check. Resource visibility uses
  the same string 'team' in a separate domain (Tree/StepLibrary
  visibility) and is intentionally untouched.
- New backend/scripts/sync_stripe_plan_ids.py: idempotent upsert of
  plan_billing rows from Stripe products by exact name match. Picks
  the active monthly recurring price for tiers that have one; leaves
  annual fields NULL by design. Works against test or live keys.
- Test fixture updates: conftest seeds the new taxonomy, the public
  plans helper is a true upsert so tests can override max_users, and
  team -> enterprise across test_admin_plan_limits and test_invite_plan.

Verified: 86/86 passing across the subscription/billing/plan/invite/
admin sweep; sync script run against test mode populates plan_billing
correctly for all three tiers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/test_plans_public.py

@@ -14,7 +14,12 @@ from app.models.plan_limits import PlanLimits
 
 
 async def _seed_plan_limits(test_db, plan: str, max_users: int | None) -> None:
-    """Ensure a plan_limits row exists for the given plan name."""
+    """Ensure a plan_limits row exists with the given max_users.
+
+    Upserts: conftest seeds the canonical plans (free/starter/pro/enterprise)
+    so this helper has to overwrite max_users when a test wants different
+    values for fixture-driven assertions.
+    """
     existing = await test_db.get(PlanLimits, plan)
     if existing is None:
         test_db.add(
@@ -28,7 +33,9 @@ async def _seed_plan_limits(test_db, plan: str, max_users: int | None) -> None:
                 export_formats=["markdown", "text"],
             )
         )
-        await test_db.commit()
+    else:
+        existing.max_users = max_users
+    await test_db.commit()
 
 
 class TestGetPlansPublic: