sfm_webapp: harmonize "Received by server at" → "Time received"

Matches Terra-View's event-modal relabel from the same iteration.
Wording was already clearer here than in Terra-View's "Captured at",
but using identical text across both surfaces means operators see the
same label whether they're in the native modal or the standalone
webapp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sfm/sfm_webapp.html

@@ -3287,7 +3287,7 @@ if (currentSection === 'db') {
           <dt id="sc-l-bwsize">File size</dt>   <dd id="sc-f-bwsize">—</dd>
           <dt id="sc-l-sha">File sha256</dt>    <dd id="sc-f-sha">—</dd>
           <dt>Source kind</dt>      <dd id="sc-f-src">—</dd>
-          <dt title="When our server received and stored this event (sfm-db insert time, not the recording time)">Received by server at</dt>
+          <dt title="When SFM received and stored this event — NOT the unit-local trigger time (see Timestamp at the top of the modal for that).">Time received</dt>
                                     <dd id="sc-f-cap">—</dd>
         </dl>
       </div>