feat(quotas): limit resolution and the quota service

docsgpt/quotas resolves a user's effective token and cost limits from the
policy rows that apply to them: their own override, then the most generous
allowance among their teams, then the instance default, then any defaults a
registered provider supplies. Each budget resolves on its own, and a team
counts once however many memberships the user holds in it.

QuotaService compares those limits with the user's token_usage totals over
the current QUOTA_PERIOD window (calendar-aligned, UTC, computed at read
time). It fails open, and skips the usage query for unlimited users.

docsgpt/quotas/__init__.py

@@ -0,0 +1,24 @@
+"""Admin-set usage quotas.
+
+Limits live in ``quota_policies`` at three layers (instance default, team
+per-member allowance, user override), each with a token budget and a USD
+budget. ``QuotaService`` resolves a user's effective limits and compares them
+with their ``token_usage`` totals over the current ``QUOTA_PERIOD`` window.
+"""
+
+from docsgpt.quotas.providers import QuotaDefaultsProvider, register_defaults_provider
+from docsgpt.quotas.resolver import ResolvedLimit, ResolvedLimits, resolve_limits
+from docsgpt.quotas.service import BucketStatus, QuotaExceeded, QuotaService
+from docsgpt.quotas.windows import window_bounds
+
+__all__ = [
+    "BucketStatus",
+    "QuotaDefaultsProvider",
+    "QuotaExceeded",
+    "QuotaService",
+    "ResolvedLimit",
+    "ResolvedLimits",
+    "register_defaults_provider",
+    "resolve_limits",
+    "window_bounds",
+]