Optional description field on activities

A free-text body alongside title/tags/location/scheduled. Plain text
for now; markdown rendering is a deliberate non-goal (the user noted
it was nice-to-have but not essential).

Schema (additive, idempotent via ensureColumn):
  - activities.description TEXT NULL
  - For private rows the column stays NULL; the description lives
    inside the encrypted payload alongside title.

Wire/types:
  - PrivatePayload.description?: string  (in shared/crypto.ts)
  - ActivityPublic.description / ActivitySemi.description: string | null
  - CreateActivityRequest.description?: string | null

Server:
  - INSERT and UPDATE handlers now write description for semi/public
  - Private→semi/public transition: description column populated
  - Semi/public→private transition: description column wiped (now in
    the encrypted blob)
  - serialize() includes the column on public and semi rows
  - server/users.ts public-list endpoint surfaces it too

Frontend:
  - ActivityForm.svelte: textarea after the title field; round-trips
    through the existing private-encrypt / plaintext-PATCH paths
  - ActivityRow.svelte: renders the description as a `white-space:
    pre-wrap` <p> so line breaks survive without enabling markdown
  - Home.svelte: search now matches against the description text
    (decrypted client-side for private rows, just like the title)

frontend/src/components/ActivityForm.svelte

@@ -45,6 +45,14 @@
       : '',
   );
   // svelte-ignore state_referenced_locally
+  let description = $state(
+    existing
+      ? (existing.visibility === 'private'
+          ? initialPriv?.description ?? ''
+          : existing.description ?? '')
+      : '',
+  );
+  // svelte-ignore state_referenced_locally
   let tags: string[] = $state(
     existing
       ? (existing.visibility === 'private' ? initialPriv?.tags ?? [] : [...existing.tags])
@@ -103,6 +111,7 @@
       const payload: PrivatePayload = {
         title: title.trim(),
         tags,
+        description: description.trim() || undefined,
         loc_label: locLabel || undefined,
         scheduled_at: scheduledEpoch() ?? undefined,
       };
@@ -116,6 +125,7 @@
     return {
       visibility,
       title: title.trim(),
+      description: description.trim() || null,
       tags,
       loc_label: locLabel || null,
       scheduled_at: scheduledEpoch(),
@@ -195,6 +205,10 @@
   <label for="title">Tittel</label>
   <input id="title" type="text" bind:value={title} required />
 
+  <label for="desc">Beskrivelse (valgfritt)</label>
+  <textarea id="desc" rows="4" bind:value={description}
+            placeholder="Detaljer, lenker, hva som helst"></textarea>
+
   <div id="tags-label" style="margin: 0.75rem 0 0.25rem; font-weight: 500;">Etiketter</div>
   <div role="group" aria-labelledby="tags-label">
     <TagInput visibility={visibility} bind:tags onChange={(t) => (tags = t)} />

frontend/src/components/ActivityRow.svelte

@@ -181,6 +181,9 @@
         </a>
         <span class="vis-badge private">Privat</span>
       </h3>
+      {#if decrypted.description}
+        <p style="white-space: pre-wrap; margin: 0.25rem 0;">{decrypted.description}</p>
+      {/if}
       {#if decrypted.tags.length}
         <div>
           {#each decrypted.tags as t}<span class="tag private">{t}</span>{/each}
@@ -202,6 +205,9 @@
         {activity.visibility === 'semi' ? 'Anonym' : 'Offentlig'}
       </span>
     </h3>
+    {#if activity.description}
+      <p style="white-space: pre-wrap; margin: 0.25rem 0;">{activity.description}</p>
+    {/if}
     {#if activity.tags.length}
       <div>
         {#each activity.tags as t}<span class="tag">{t}</span>{/each}

frontend/src/components/Home.svelte

@@ -84,14 +84,15 @@
     if (a.visibility === 'private') {
       const p = privateCleartext.get(a.id);
       if (!p) return false;
-      return [p.title, p.loc_label ?? '', ...p.tags]
+      return [p.title, p.description ?? '', p.loc_label ?? '', ...p.tags]
         .some((s) => s.toLowerCase().includes(needle));
     }
     return [
       a.title,
+      a.description ?? '',
       a.loc_label ?? '',
       ...a.tags,
-      a.visibility === 'public' ? a.owner_display : '',
+      a.visibility === 'public' ? a.owner_display ?? '' : '',
     ].some((s) => s.toLowerCase().includes(needle));
   }