Audit round 2: aspect measured the case too; defensive image decode; own-key state reset

4. aspect_profile had the same root cause as edges/centering but no guard.
   On the cased photo it reported 5.8% off standard -- the CASE's
   proportions -- and the UI would surface that as a trimming hint about a
   card it never measured. This one is more directly load-bearing than the
   other two, since the measurement IS the detected box. Now refuses, with
   both consumers (prompt block and UI hint) checking reliable.

5. _decode_images could KeyError/raise out of a plain read on a malformed
   row, and separately b64decode returns b'' rather than raising on some
   corrupt input -- which would have fed a zero-byte 'photo' into grading
   to fail confusingly deep in the pipeline. Both now degrade to the
   existing re-pick path.

6. _last_used_own_key is instance state on a handler that serves every
   request on a keep-alive connection, so it outlives the request that set
   it. Currently safe (every logging path assigns first), but a future path
   that logged without reaching the assignment would bill the previous
   request's payer. Cleared up-front now.

Full regression suite re-run: die-cut exclusion, uniform cards, centering,
aspect, cased-photo refusals, and the store layer all still behave.

vision.py

@@ -951,7 +951,7 @@ def grade_card(images, api_key=None, model=None, effort=None, zoom_details=True)
                 "strip images to describe what the wear looks like and to "
                 "catch what the measurement does not look for at all, such as "
                 "a nick, a chip, or a crushed edge.")
-    if aspect:
+    if aspect and aspect.get("reliable") is not False:
         standard_rows = "\n".join(
             "  {}: {:.4f} standard → {:.1f}% deviation".format(
                 name, data["standard_ratio"], data["deviation_percent"])