feat: immutable entries + full task logging

Entries now immutable once journal is generated:
- Edit/delete returns ENTRY_IMMUTABLE error if journal exists
- Frontend shows lock message and hides delete button
- Delete Journal button to unlock entries

Task logging now stores full JSON:
- request: full JSON request sent to AI provider
- response: full JSON response from AI provider
- prompt: formatted human-readable prompt

Prompt structure:
1. System prompt
2. Previous diary entries (journals)
3. Today's entries

frontend/src/components/EntryList.tsx

@@ -3,9 +3,10 @@ import type { Entry } from '../lib/api';
 interface Props {
   entries: Entry[];
   onDelete: (id: string) => void;
+  readOnly?: boolean;
 }
 
-export default function EntryList({ entries, onDelete }: Props) {
+export default function EntryList({ entries, onDelete, readOnly }: Props) {
   const formatTime = (dateStr: string) => {
     return new Date(dateStr).toLocaleTimeString('en-US', { hour: 'numeric', minute: '2-digit' });
   };
@@ -34,6 +35,11 @@ export default function EntryList({ entries, onDelete }: Props) {
 
   return (
     <div className="space-y-3">
+      {readOnly && (
+        <div className="bg-amber-500/10 border border-amber-500/30 rounded-lg p-3 text-sm text-amber-400">
+          🔒 Entries are locked because a journal has been generated. Delete the journal to edit entries.
+        </div>
+      )}
       {entries.map((entry) => (
         <div
           key={entry.id}
@@ -63,12 +69,14 @@ export default function EntryList({ entries, onDelete }: Props) {
                 </div>
               )}
             </div>
-            <button
-              onClick={() => onDelete(entry.id)}
-              className="text-slate-500 hover:text-red-400 text-sm transition"
-            >
-              ×
-            </button>
+            {!readOnly && (
+              <button
+                onClick={() => onDelete(entry.id)}
+                className="text-slate-500 hover:text-red-400 text-sm transition"
+              >
+                ×
+              </button>
+            )}
           </div>
         </div>
       ))}

frontend/src/pages/Day.tsx

@@ -9,6 +9,7 @@ export default function Day() {
   const [entries, setEntries] = useState<Entry[]>([]);
   const [loading, setLoading] = useState(true);
   const [hasJournal, setHasJournal] = useState(false);
+  const [deleteError, setDeleteError] = useState<string | null>(null);
 
   useEffect(() => {
     if (date) loadEntries();
@@ -35,9 +36,21 @@ export default function Day() {
   };
 
   const handleDeleteEntry = async (id: string) => {
+    setDeleteError(null);
     const res = await api.deleteEntry(id);
     if (res.data) {
       setEntries((prev) => prev.filter((e) => e.id !== id));
+    } else if (res.error?.code === 'ENTRY_IMMUTABLE') {
+      setDeleteError(res.error.message);
+    }
+  };
+
+  const handleDeleteJournal = async () => {
+    if (!date) return;
+    if (!confirm('Delete journal? This will unlock entries for editing.')) return;
+    const res = await api.deleteDay(date);
+    if (res.data) {
+      setHasJournal(false);
     }
   };
 
@@ -56,12 +69,26 @@ export default function Day() {
           <h1 className="text-2xl font-bold">{formatDate(date)}</h1>
         </div>
         {hasJournal && (
-          <a href={`/journal/${date}`} className="px-4 py-2 bg-purple-600 hover:bg-purple-700 rounded-lg font-medium transition">
-            View Journal
-          </a>
+          <div className="flex gap-2">
+            <button
+              onClick={handleDeleteJournal}
+              className="px-3 py-2 bg-red-600/20 hover:bg-red-600/30 text-red-400 rounded-lg text-sm transition"
+            >
+              Delete Journal
+            </button>
+            <a href={`/journal/${date}`} className="px-4 py-2 bg-purple-600 hover:bg-purple-700 rounded-lg font-medium transition">
+              View Journal
+            </a>
+          </div>
         )}
       </div>
 
+      {deleteError && (
+        <div className="bg-red-500/20 border border-red-500/30 rounded-lg p-3 mb-4 text-sm text-red-400">
+          {deleteError}
+        </div>
+      )}
+
       <EntryInput onSubmit={handleAddEntry} />
 
       {loading ? (
@@ -71,7 +98,7 @@ export default function Day() {
           <p className="text-slate-400">No entries for this day</p>
         </div>
       ) : (
-        <EntryList entries={entries} onDelete={handleDeleteEntry} />
+        <EntryList entries={entries} onDelete={handleDeleteEntry} readOnly={hasJournal} />
       )}
     </div>
   );