fix: bump enterprise function limit from 200k to 500k

Real-world codebase (effect-ts) has 200,437 functions -- just 437 over
the old 200,000 cap. Enterprise tier should handle large codebases
without arbitrary limits causing friction.

289 tests pass.

Author: DevanshuNEU

backend/services/user_limits.py

@@ -66,7 +66,7 @@ class TierLimits:
     UserTier.ENTERPRISE: TierLimits(
         max_repos=None,  # Unlimited
         max_files_per_repo=50000,
-        max_functions_per_repo=200000,
+        max_functions_per_repo=500000,
         playground_searches_per_day=None,
         max_team_members=None,
         priority_indexing=True,