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/services/ai/provider.ts

@@ -1,6 +1,12 @@
+export interface AIProviderResult {
+  content: string;
+  request: Record<string, unknown>;
+  response: Record<string, unknown>;
+}
+
 export interface AIProvider {
   provider: 'openai' | 'anthropic' | 'ollama' | 'lmstudio' | 'groq';
-  generate(prompt: string, systemPrompt?: string): Promise<string>;
+  generate(prompt: string, systemPrompt?: string): Promise<AIProviderResult>;
   validate?(): Promise<boolean>;
 }