Add tier-gated RAG and Drive document sources (#42)

Ship allows_rag entitlement (founders/backer/pro/business), enforce it on
document APIs and RAG chat, harden ingest/delete/active lifecycle, and add
Google/Microsoft Drive connect + sync for personal and company knowledge bases.

Closes #43 #44 #45 #46 #47 #48 #49 #50 #51 #52 #53
Parent epic: #42
Related: #11

llm_be/finance/models.py

@@ -40,6 +40,10 @@ class SubscriptionPlan(TimeInfoBase):
     )
     allows_text_generation = models.BooleanField(default=True)
     allows_image_generation = models.BooleanField(default=False)
+    allows_rag = models.BooleanField(
+        default=False,
+        help_text="Drive/RAG document sync (Google Drive, OneDrive, SharePoint).",
+    )
     allows_all_future_features = models.BooleanField(
         default=False,
         help_text="Founders/Backer: unlock new capabilities as they ship.",
@@ -74,6 +78,8 @@ class SubscriptionPlan(TimeInfoBase):
             return self.allows_text_generation
         if feature in ("image", "image_generation"):
             return self.allows_image_generation
+        if feature in ("rag", "document_rag"):
+            return self.allows_rag
         return False