feat(db): add system settings queries and enhance user queries

System Settings Migration:
- Add GetSystemSetting query for single setting retrieval
- Add UpdateSystemSetting query for updating settings
- Add GetAllSystemSettings query for all settings
- Remove UpdateScanSettings and GetScanSettings (per-user queries)

User Query Enhancement:
- Add max_devices field to GetUser query
- Add device_count computed field to GetUser query
- Add max_devices field to ListUsers query
- Add device_count computed field to ListUsers query

These changes support:
1. System-wide scan settings instead of per-user settings
2. Users can now see their device count and limits
3. Admins can monitor device usage across all users

internal/database/queries/queries.sql

@@ -16,13 +16,39 @@ SELECT id, email, username, theme, first_name, last_name, role, created_at, upda
 SELECT id, email, username, password_hash, theme, first_name, last_name, role, created_at, updated_at FROM users WHERE email = $1 OR username = $1;
 
 -- name: GetUser :one
-SELECT id, email, username, theme, first_name, last_name, role, created_at, updated_at FROM users WHERE id = $1;
+SELECT 
+    u.id,
+    u.email,
+    u.username,
+    u.theme,
+    u.first_name,
+    u.last_name,
+    u.role,
+    u.max_devices,
+    u.created_at,
+    u.updated_at,
+    (SELECT COUNT(*) FROM devices WHERE user_id = u.id) as device_count
+FROM users u
+WHERE u.id = $1;
 
 -- name: GetUserPasswordHash :one
 SELECT password_hash FROM users WHERE id = $1;
 
 -- name: ListUsers :many
-SELECT id, email, username, theme, first_name, last_name, role, created_at, updated_at FROM users ORDER BY created_at DESC;
+SELECT 
+    u.id,
+    u.email,
+    u.username,
+    u.theme,
+    u.first_name,
+    u.last_name,
+    u.role,
+    u.max_devices,
+    u.created_at,
+    u.updated_at,
+    (SELECT COUNT(*) FROM devices WHERE user_id = u.id) as device_count
+FROM users u
+ORDER BY u.created_at DESC;
 
 -- Library Types queries
 -- name: GetLibraryTypes :many
@@ -317,11 +343,15 @@ SELECT COUNT(*) FROM devices WHERE user_id = $1;
 -- name: DeleteUser :exec
 DELETE FROM users WHERE id = $1;
 
--- name: UpdateScanSettings :exec
-UPDATE users SET scan_frequency_minutes = $2, auto_scan_enabled = $3, updated_at = NOW() WHERE id = $1;
+-- System Settings queries
+-- name: GetSystemSetting :one
+SELECT setting_value FROM system_settings WHERE setting_key = $1;
 
--- name: GetScanSettings :one
-SELECT scan_frequency_minutes, auto_scan_enabled FROM users WHERE id = $1;
+-- name: UpdateSystemSetting :exec
+UPDATE system_settings SET setting_value = $2, updated_at = NOW() WHERE setting_key = $1;
+
+-- name: GetAllSystemSettings :many
+SELECT setting_key, setting_value, description FROM system_settings ORDER BY setting_key;
 
 -- name: CreateMediaRating :one
 INSERT INTO media_ratings (media_item_id, user_id, rating)