Add LLM settings integration

Add app_settings migration, settings UI, and OpenAI-compatible httpx LLM client with mocked tests.

Preserve API keys on blank form submissions, require a fresh key when base_url changes, and keep AI search settings untouched for step 3.

Update docs/design LLM integration and step 3 AI search notes, including prompt contract and extra-hints planning.

app/settings_store.py

@@ -0,0 +1,88 @@
+"""Settings read/write helpers for the ``app_settings`` KV table.
+
+Provides a typed ``LLMConfig`` dataclass and two helpers:
+
+- ``get_app_settings(db) -> LLMConfig`` — reads KV rows (or returns defaults).
+- ``save_app_settings(db, ...) -> None`` — writes KV rows; API key left blank
+  means "keep the old value".
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from sqlalchemy.orm import Session
+
+from app.models import AppSetting
+
+
+@dataclass
+class LLMConfig:
+    """All settings consumed by the LLM client and settings UI."""
+
+    enabled: bool = False
+    base_url: str = "https://api.openai.com/v1"
+    model: str = ""
+    api_key: str = ""
+    ai_search_enabled: bool = False
+
+
+def _get_value(rows: dict[str, str], key: str, default: str) -> str:
+    return rows.get(key, default)
+
+
+def _get_bool(rows: dict[str, str], key: str, default: bool) -> bool:
+    return rows.get(key, str(default).lower()) == "true"
+
+
+def get_app_settings(db: Session) -> LLMConfig:
+    """Read all settings from ``app_settings`` and return an ``LLMConfig``."""
+    rows: dict[str, str] = {}
+    for row in db.query(AppSetting).all():
+        if row.value is not None:
+            rows[row.key] = row.value
+
+    return LLMConfig(
+        enabled=_get_bool(rows, "llm_enabled", False),
+        base_url=_get_value(rows, "llm_base_url", "https://api.openai.com/v1"),
+        model=_get_value(rows, "llm_model", ""),
+        api_key=_get_value(rows, "llm_api_key", ""),
+        ai_search_enabled=_get_bool(rows, "ai_search_enabled", False),
+    )
+
+
+def save_app_settings(
+    db: Session,
+    *,
+    enabled: bool | None = None,
+    base_url: str | None = None,
+    model: str | None = None,
+    api_key: str | None = None,
+    ai_search_enabled: bool | None = None,
+) -> None:
+    """Write settings to ``app_settings``.
+
+    If ``api_key`` is ``None`` (form field left blank), the existing key is
+    preserved.  All other fields are written as-is.
+    """
+    updates: dict[str, str | None] = {}
+
+    if enabled is not None:
+        updates["llm_enabled"] = str(enabled).lower()
+    if base_url is not None:
+        updates["llm_base_url"] = base_url
+    if model is not None:
+        updates["llm_model"] = model
+    if api_key is not None:
+        updates["llm_api_key"] = api_key
+    if ai_search_enabled is not None:
+        updates["ai_search_enabled"] = str(ai_search_enabled).lower()
+
+    for key, value in updates.items():
+        existing = db.get(AppSetting, key)
+        if existing is not None:
+            existing.value = value
+        else:
+            db.add(AppSetting(key=key, value=value))
+
+    db.commit()