feat: add buildUserHistory migration script

One-time script to read all User joinDates from Datastore, bucket by
month, build cumulative history, and store as Setting entity
'user_count_history'. Cost ~$0.16 (315k reads). Run once only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sspickle

ide/shell_cmds/buildUserHistory.py

@@ -0,0 +1,71 @@
+"""
+One-time migration: read all User joinDates, build monthly cumulative history,
+store in Setting entity 'user_count_history'.
+
+Usage (production):
+    GOOGLE_CLOUD_PROJECT=glowscript python ide/shell_cmds/buildUserHistory.py
+
+Usage (local emulator):
+    DATASTORE_EMULATOR_HOST=localhost:8081 python ide/shell_cmds/buildUserHistory.py
+
+Cost: ~$0.16 (reads ~315k User entities). Run once only.
+"""
+
+from collections import defaultdict
+from datetime import datetime, timezone
+import json
+import os
+
+from google.cloud import ndb
+from ide.models import User, Setting
+
+
+def build_history(client):
+    monthly_new = defaultdict(int)
+    count = 0
+    skipped = 0
+
+    with client.context():
+        for user in User.query():  # NDB iterates in batches — does not load all into memory
+            count += 1
+            if count % 10000 == 0:
+                print(f"  {count} users processed...")
+
+            join_date = getattr(user, 'joinDate', None)
+            if join_date:
+                month_str = join_date.strftime('%Y-%m')
+                monthly_new[month_str] += 1
+            else:
+                skipped += 1
+
+        print(f"Done: {count} users, {skipped} skipped (no joinDate)")
+
+        sorted_months = sorted(monthly_new.keys())
+        cumulative = 0
+        points = []
+        for month in sorted_months:
+            cumulative += monthly_new[month]
+            points.append({'month': month, 'count': cumulative})
+
+        history = {
+            'updated': datetime.now(timezone.utc).strftime('%Y-%m-%d'),
+            'points': points,
+        }
+
+        existing = ndb.Key('Setting', 'user_count_history').get()
+        if not existing:
+            existing = Setting(id='user_count_history')
+        existing.value = json.dumps(history)
+        existing.put()
+
+        print(f"Stored {len(points)} monthly data points")
+        if points:
+            print(f"Latest: {points[-1]}")
+
+
+if __name__ == '__main__':
+    project = os.environ.get('GOOGLE_CLOUD_PROJECT', 'glowscript')
+    emulator = os.environ.get('DATASTORE_EMULATOR_HOST')
+    client = ndb.Client(project='glowscript-dev' if emulator else project)
+    print(f"Connecting to {'emulator at ' + emulator if emulator else 'production Datastore'}...")
+    build_history(client)