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/index.html

@@ -74,6 +74,8 @@
           <th>Person B</th>
           <th>Start</th>
           <th>Ende</th>
+          <th>Anlass</th>
+          <th>Endgrund</th>
           <th>Aktion</th>
           <th></th>
         </tr>
@@ -100,6 +102,14 @@
         <input id="f-tsend" type="text" maxlength="120"
                placeholder="z. B. laufend, unbekannt, März 2026 …" />
       </label>
+      <label>Anlass (optional)
+        <input id="f-startcause" type="text" maxlength="300"
+               placeholder="womit/warum es begann, z. B. gemeinsam geplante Reise …" />
+      </label>
+      <label>Endgrund (optional)
+        <input id="f-endcause" type="text" maxlength="300"
+               placeholder="warum es endete, z. B. zurückgezahlt, abgelaufen …" />
+      </label>
       <label>Aktion
         <input id="f-action" type="text" required maxlength="500" />
       </label>