Fix bugs, N+1 queries, and wire settings in persian-tutor

- Replace inline __import__("datetime").timedelta hack with proper import
- Remove unused import random in anki_export.py
- Add error handling for Claude CLI subprocess failures in ai.py
- Fix hardcoded absolute path in stt.py with relative Path resolution
- Fix N+1 DB queries in vocab.get_flashcard_batch and dashboard.get_category_breakdown
  by adding db.get_all_word_progress() batch query
- Wire Ollama model and Whisper size settings to actually update config
  via ai.set_ollama_model() and stt.set_whisper_size()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/persian-tutor/ai.py

@@ -6,9 +6,18 @@ import ollama
 
 DEFAULT_OLLAMA_MODEL = "qwen2.5:7b"
 
+_ollama_model = DEFAULT_OLLAMA_MODEL
 
-def ask_ollama(prompt, system=None, model=DEFAULT_OLLAMA_MODEL):
+
+def set_ollama_model(model):
+    """Change the Ollama model used for fast queries."""
+    global _ollama_model
+    _ollama_model = model
+
+
+def ask_ollama(prompt, system=None, model=None):
     """Query Ollama with an optional system prompt."""
+    model = model or _ollama_model
     messages = []
     if system:
         messages.append({"role": "system", "content": system})
@@ -24,6 +33,8 @@ def ask_claude(prompt):
         capture_output=True,
         text=True,
     )
+    if result.returncode != 0:
+        raise RuntimeError(f"Claude CLI failed (exit {result.returncode}): {result.stderr.strip()}")
     return result.stdout.strip()
 
 
@@ -34,8 +45,9 @@ def ask(prompt, system=None, quality="fast"):
     return ask_ollama(prompt, system=system)
 
 
-def chat_ollama(messages, system=None, model=DEFAULT_OLLAMA_MODEL):
+def chat_ollama(messages, system=None, model=None):
     """Multi-turn conversation with Ollama."""
+    model = model or _ollama_model
     all_messages = []
     if system:
         all_messages.append({"role": "system", "content": system})