feat: free-text timestamps instead of datetime pickers

Users wanted to type things like "unbekannt" or comments for the date
fields. Start/End are now plain text inputs (max 120 chars) and are
stored/displayed verbatim - no Date parsing, no formatting. Existing
records that contain ISO strings keep working: they render exactly as
stored and refill the edit form unchanged. Also removes a duplicated
block in setup() that had slipped in earlier (double reload/flash).

Verified: node --check, no datetime-local/Date-parsing remnants,
crypto roundtrip with free-text and legacy ISO timestamps, pytest 3/3.

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

backend/static/index.html

@@ -92,11 +92,13 @@
       <label>Person B
         <input id="f-personb" type="text" required maxlength="120" />
       </label>
-      <label>Start (optional)
-        <input id="f-tsstart" type="datetime-local" />
+      <label>Start (optional, beliebiger Text)
+        <input id="f-tsstart" type="text" maxlength="120"
+               placeholder="z. B. 12.03.2026, Sommer 2025, unbekannt …" />
       </label>
-      <label>Ende (optional)
-        <input id="f-tsend" type="datetime-local" />
+      <label>Ende (optional, beliebiger Text)
+        <input id="f-tsend" type="text" maxlength="120"
+               placeholder="z. B. laufend, unbekannt, März 2026 …" />
       </label>
       <label>Aktion
         <input id="f-action" type="text" required maxlength="500" />