Reorganize core into subpackages, add local KB, ruff/pyright tooling

- Split backend/app/core into weather/, discord/, memory_kb/, search/
  subpackages by responsibility, keeping config.py and llm.py as the
  shared foundation; updated all import sites accordingly.
- Add local Markdown knowledge base support (local_knowledge.py,
  knowledge_base/fgo.md, LOCAL_KB_* settings) that embeds and injects
  relevant docs into the system prompt by topic similarity.
- Add ruff and pyright as backend dev dependencies with basic config
  in pyproject.toml for linting and type checking.
- Document local machine quirks in CLAUDE.md (Postgres on port 8080,
  SSLKEYLOGFILE breaking litellm/pyright via OPENSSL_Uplink crashes).
- Add backend/run_dev.ps1 dev launcher and docs/discord-bot.md.

backend/app/core/weather/weather_cache.py

@@ -0,0 +1,19 @@
+"""直近で取得した「明日の天気」データをメモリ上にキャッシュする。"""
+from datetime import datetime
+
+_latest: dict | None = None
+
+
+def set_latest(weather: str, pop: int | None, judgement: str, reason: str) -> None:
+    global _latest
+    _latest = {
+        "weather": weather,
+        "pop": pop,
+        "judgement": judgement,
+        "reason": reason,
+        "fetched_at": datetime.now().isoformat(),
+    }
+
+
+def get_latest() -> dict | None:
+    return _latest