feat: Anlass + Endgrund fields for records

Adds two optional free-text fields to describe why a transaction
started ("Anlass") and why it ended ("Endgrund") - chosen names instead
of the technical "Startursache"/"Endesuhrsache". Displayed as own table
columns and editable in the form; legacy records without those fields
render as "-". Payload is encrypted client-side like every other field
(no server change needed, opaque ciphertext).

Verified: node --check, crypto roundtrip incl. new fields, legacy-safe
access (?? ''), pytest 3/3.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

backend/static/ui.js

@@ -104,6 +104,8 @@ function renderRow(record, payload) {
     payload.personB,
     showTs(payload.tsStart),
     showTs(payload.tsEnd),
+    showTs(payload.startCause),
+    showTs(payload.endCause),
     payload.action,
   ];
   for (const text of cells) {
@@ -145,7 +147,7 @@ async function reload() {
       failures++;
       tbody.appendChild(
         el("tr", { class: "row-error" }, [
-          el("td", { colspan: "6", text: "Datensatz nicht entschlüsselbar (falsche Passphrase oder manipuliert)." }),
+          el("td", { colspan: "8", text: "Datensatz nicht entschlüsselbar (falsche Passphrase oder manipuliert)." }),
         ])
       );
     }
@@ -164,6 +166,8 @@ function openEditor(record, payload) {
   $("#f-personb").value = payload.personB ?? "";
   $("#f-tsstart").value = payload.tsStart ?? "";
   $("#f-tsend").value = payload.tsEnd ?? "";
+  $("#f-startcause").value = payload.startCause ?? "";
+  $("#f-endcause").value = payload.endCause ?? "";
   $("#f-action").value = payload.action ?? "";
   $("#editor-backdrop").hidden = false;
 }
@@ -201,6 +205,8 @@ async function saveRecord(ev) {
     personB: $("#f-personb").value.trim(),
     tsStart: tsOrNull($("#f-tsstart").value),
     tsEnd: tsOrNull($("#f-tsend").value),
+    startCause: tsOrNull($("#f-startcause").value),
+    endCause: tsOrNull($("#f-endcause").value),
     action: $("#f-action").value.trim(),
   };
   if (!payload.personA || !payload.personB || !payload.action) {