Add Regrade: instant re-run when photos are stored, fallback re-pick otherwise

Stores the original photo(s) for every card graded from now on, so an
existing history row can be re-graded in place without re-uploading -- most
useful right after a grading-logic fix like the last few commits. Cards
graded before this feature (or saved without images) fall back to the same
photo picker as a fresh grade, then start getting instant regrades from then
on. list_grades/get_grade now select an explicit column list rather than
SELECT * so the stored images are never pulled into memory just to be
discarded for the browser-facing response.

static/index.html

@@ -61,6 +61,11 @@
     <div id="grade-review"></div>
   </section>
 
+  <!-- Fallback photo picker for Regrade on a card saved before this feature
+       existed (or otherwise missing its stored photo) — regradeTargetId in
+       app.js tracks which row this fires for. -->
+  <input id="regrade-file" type="file" accept="image/*" multiple hidden>
+
   <!-- ----------------------------------------------------------- history -->
   <section class="panel">
     <div class="panel-head">