refactor: consolidate isUnique and path validation helpers (Task 28)

- seed.Admin: hand-rolled pgconn.PgError 23505 check → store.IsUniqueViolation
  (single predicate for unique violations across the codebase)
- books.absBookPath: local hasPrefixDir (filepath.Rel-based) removed in favor
  of bookfile.Contains, which also resolves symlinks — stricter escape check
- handlers isUnique alias + libraries.go local prefix check were already
  folded into ports.IsUniqueViolation / bookfile.Contains in Task 25;
  scanner.inside was folded in Task 24 — this commit closes the last two

Full gate green: gofmt, vet, go test -p 1 (real PG+Redis, 0 skip)

backend/internal/seed/seed.go

@@ -2,11 +2,8 @@ package seed
 
 import (
 	"context"
-	"errors"
 	"log"
 
-	"github.com/jackc/pgx/v5/pgconn"
-
 	"booklib/internal/auth"
 	"booklib/internal/store"
 )
@@ -28,8 +25,7 @@ func Admin(ctx context.Context, s *store.Store, user, pass string) error {
 		return err
 	}
 	if _, err := s.CreateUser(ctx, user, h, "admin"); err != nil {
-		var pgErr *pgconn.PgError
-		if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+		if store.IsUniqueViolation(err) {
 			log.Printf("seed admin %q may already exist: %v", user, err)
 			return nil
 		}