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.

cardimage.py

@@ -939,6 +939,16 @@ def aspect_profile(image_bytes):
         bw, bh = box[2] - box[0], box[3] - box[1]
         if bw < 40 or bh < 40:
             return None
+        # This measurement IS the detected box, so a box that didn't find
+        # the card's real boundary doesn't produce an approximate ratio —
+        # it produces the ratio of something else entirely. On a cased card
+        # it reported 5.8% off standard, which is the CASE's proportions;
+        # the UI would then show a trimming hint about a card it never
+        # measured. Same guard as edge/centering, and more directly
+        # load-bearing here than for either of those.
+        margin_reason = _box_margin_reason(box, img.size)
+        if margin_reason:
+            return {"reliable": False, "reason": margin_reason}
         ratio = min(bw, bh) / float(max(bw, bh))
 
         against_standards = {
@@ -951,6 +961,7 @@ def aspect_profile(image_bytes):
         best_name = min(against_standards, key=lambda n: against_standards[n]["deviation_percent"])
 
         return {
+            "reliable": True,
             "measured_ratio": round(ratio, 4),
             "width_px": bw,
             "height_px": bh,