Fix timezone mismatch and OAuth profile field access

- Add getLatestDateWithData() helper that queries the most recent date
  with price data instead of using server UTC time
- Cache latest date for 5 minutes to reduce database queries
- Update all date-dependent endpoints to use database-relative dates
- Fix OAuth callback profile field access (use profile.email instead
  of profile.emails[0].value)
- Remove loginCount from $setOnInsert to avoid MongoDB conflict error

models/user.js

@@ -18,21 +18,20 @@ async function findOrCreateUser(db, profile) {
   const now = new Date();
 
   const result = await db.collection(USER_COLLECTION).findOneAndUpdate(
-    { googleId: profile.id },
+    { googleId: profile.googleId },
     {
       $set: {
-        email: profile.emails[0].value,
-        name: profile.displayName,
-        picture: profile.photos?.[0]?.value || null,
+        email: profile.email,
+        name: profile.name,
+        picture: profile.picture || null,
         lastLoginAt: now
       },
       $inc: { loginCount: 1 },
       $setOnInsert: {
-        googleId: profile.id,
+        googleId: profile.googleId,
         isActive: true,
         role: 'user',
-        createdAt: now,
-        loginCount: 0 // Will be incremented to 1 by $inc
+        createdAt: now
       }
     },
     {