fix(api): classify DB errors in library paths

backend/internal/api/libraries.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"context"
+	"errors"
 	"io"
 	"net/http"
 	"os"
@@ -11,6 +12,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5"
 
 	"booklib/internal/bookfile"
 	"booklib/internal/store"
@@ -60,7 +62,7 @@ func (a *api) createLibrary(c *gin.Context) {
 			err(c, http.StatusConflict, "exists", "root_path taken")
 			return
 		}
-		err(c, http.StatusBadRequest, "bad_request", "invalid input")
+		dbErr(c, e)
 		return
 	}
 	c.JSON(http.StatusCreated, gin.H{"id": id, "name": req.Name, "root_path": filepath.Clean(req.RootPath)})
@@ -74,7 +76,11 @@ func (a *api) getLibrary(c *gin.Context) (store.Library, bool) {
 	}
 	lib, e := a.st.GetLibrary(c, id)
 	if e != nil {
-		err(c, http.StatusNotFound, "not_found", "no such library")
+		if errors.Is(e, pgx.ErrNoRows) {
+			err(c, http.StatusNotFound, "not_found", "no such library")
+			return store.Library{}, false
+		}
+		dbErr(c, e)
 		return store.Library{}, false
 	}
 	return lib, true