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

backend/src/routes/entries.ts

@@ -65,6 +65,11 @@ entriesRoutes.put('/:id', async (c) => {
     return c.json({ data: null, error: { code: 'NOT_FOUND', message: 'Entry not found' } }, 404);
   }
 
+  const journal = await prisma.journal.findFirst({ where: { userId, date: existing.date } });
+  if (journal) {
+    return c.json({ data: null, error: { code: 'ENTRY_IMMUTABLE', message: 'Cannot edit entry: journal already generated. Delete the journal first.' } }, 400);
+  }
+
   const entry = await prisma.entry.update({
     where: { id },
     data: {
@@ -89,6 +94,11 @@ entriesRoutes.delete('/:id', async (c) => {
     return c.json({ data: null, error: { code: 'NOT_FOUND', message: 'Entry not found' } }, 404);
   }
 
+  const journal = await prisma.journal.findFirst({ where: { userId, date: existing.date } });
+  if (journal) {
+    return c.json({ data: null, error: { code: 'ENTRY_IMMUTABLE', message: 'Cannot delete entry: journal already generated. Delete the journal first.' } }, 400);
+  }
+
   await prisma.entry.delete({ where: { id } });
 
   return c.json({ data: { deleted: true }, error: null });