SECURITY FIX: Eliminate privilege escalation via database error exploitation

Critical Vulnerability Fixed:
- Database errors during user count check resulted in automatic admin privileges
- Any user could potentially gain admin access by triggering DB failures
- Affected both regular user registration and OIDC user creation

Root Cause Analysis:
```typescript
} catch (e) {
  isFirstUser = true;  // 💀 DANGEROUS: DB error = admin privileges
```

Linus-Style Solution - Fail Secure:
✅ Database error = reject request (don't guess permissions)
✅ Legitimate first user still gets admin (when DB works correctly)
✅ Attackers cannot exploit DB failures for privilege escalation
✅ Clear error logging for debugging

Security Impact:
- BEFORE: Database DoS → privilege escalation attack vector
- AFTER: Database error → secure rejection, no privilege guessing

Files Modified:
• users.ts:221 - Fixed user registration privilege escalation
• users.ts:670 - Fixed OIDC user creation privilege escalation

"When in doubt, fail secure. Don't guess privileges." - Security Engineering 101

src/backend/database/routes/users.ts

@@ -218,12 +218,15 @@ router.post("/create", async (req, res) => {
         .get();
       isFirstUser = ((countResult as any)?.count || 0) === 0;
     } catch (e) {
-      isFirstUser = true;
-      authLogger.warn("Failed to check user count, assuming first user", {
+      // SECURITY: Database error - fail secure, don't guess permissions
+      authLogger.error("Database error during user count check - rejecting request", {
         operation: "user_create",
         username,
         error: e,
       });
+      return res.status(500).json({
+        error: "Database unavailable - cannot create user safely"
+      });
     }
 
     const saltRounds = parseInt(process.env.SALT || "10", 10);
@@ -664,7 +667,13 @@ router.get("/oidc/callback", async (req, res) => {
           .get();
         isFirstUser = ((countResult as any)?.count || 0) === 0;
       } catch (e) {
-        isFirstUser = true;
+        // SECURITY: Database error during OIDC user creation - fail secure
+        authLogger.error("Database error during OIDC user count check", {
+          operation: "oidc_user_create",
+          oidc_identifier: identifier,
+          error: e,
+        });
+        throw new Error("Database unavailable - cannot create OIDC user safely");
       }
 
       const id = nanoid();