fix: wrap AssignRoles in transaction and eliminate N+1 queries

- AssignRoles: wrap DeleteByUserID + BatchCreate in DB transaction (P1)
- GetUserRoles: use GetByIDs batch query instead of per-role GetByID loop (N+1 fix)
- ListAdmins: use GetByIDs batch query instead of per-user GetByID loop (N+1 fix)
- Add WithTx/DB methods to UserRoleRepository for transaction support
- Add GetByIDs to UserRepository (batch user lookup)
- Add .gitattributes to normalize line endings to LF (P2)

.gitattributes

@@ -0,0 +1,32 @@
+# Normalize line endings to LF for all text files
+* text=auto eol=lf
+
+# Enforce LF for source files
+*.go text eol=lf
+*.ts text eol=lf
+*.tsx text eol=lf
+*.js text eol=lf
+*.jsx text eol=lf
+*.css text eol=lf
+*.scss text eol=lf
+*.html text eol=lf
+*.htm text eol=lf
+*.json text eol=lf
+*.yaml text eol=lf
+*.yml text eol=lf
+*.md text eol=lf
+*.sh text eol=lf
+*.ps1 text eol=lf
+*.mjs text eol=lf
+*.cjs text eol=lf
+
+# Binary files
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.pdf binary
+*.zip binary
+*.gz binary
+*.tar binary