feat: Phase B learning + hashtag profiles Pro-only lock

- Approve action saves post as reference example in character's content_rules
- Keep last 5 approved examples per character (auto-rotating)
- Inject last 3 approved examples as few-shot in LLM system prompt
- Lock YouTube/TikTok hashtag profile tabs for Freemium users (Pro only)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/content.py

@@ -77,6 +77,15 @@ def generate_post_text(
             rules_text += "\nNON FARE MAI: " + " | ".join(dont_rules)
         system_parts.append(rules_text)
 
+    # Few-shot: approved examples from past posts
+    approved_examples = content_rules.get("approved_examples", [])
+    if approved_examples:
+        examples_text = "\nESEMPI DI POST APPROVATI (usa come riferimento per stile e tono):"
+        for i, ex in enumerate(approved_examples[-3:], 1):  # last 3 in prompt
+            plat = ex.get("platform", "")
+            examples_text += f"\n--- Esempio {i} ({plat}) ---\n{ex.get('text', '')}"
+        system_parts.append(examples_text)
+
     system_parts.extend([
         "\nYou create authentic, engaging content that resonates with your audience.",
         "Never reveal you are an AI. Write as {name} would naturally write.",