Fix date-as-serial bug + cache FX lookups

append_row.py:
- Switch valueInputOption from USER_ENTERED to RAW.
- coerce() the row per-column: amount / amount_hkd / fx_rate become floats,
  everything else stays a string. Combined with RAW, dates ("2026-04-20") no
  longer get auto-parsed into Sheets date serials (e.g. 46153), while amounts
  still land as proper numeric cells so SUM/AVERAGE keep working.

fx_convert.py:
- Cache frankfurter.app responses in ~/.cache/autoacct/fx_cache.json (atomic
  write via .tmp + replace). Keyed by "<currency>_<date>". ECB historical
  rates are immutable, so an indefinite TTL is safe. Measured locally:
  cache hit 52ms vs cache miss 470ms (~9x).

setup.md: troubleshooting entries for pre-existing serial-date rows and for
the FX cache location.

Auth path also verified end-to-end via pure bash + openssl + curl (JWT
sign → token exchange → Sheets API 404 on bogus ID), proving the wire
format is correct independent of the Python client.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/fx_convert.py

@@ -13,21 +13,59 @@ from __future__ import annotations
 
 import argparse
 import json
+import os
 import sys
 import urllib.request
 from datetime import date
+from pathlib import Path
+
+# Cache file for frankfurter responses. ECB reference rates for past dates
+# never change, so an indefinite TTL is safe; same-day rates also stabilize
+# once published. Keyed by "<currency>_<on_date>" → {rate, fx_date}.
+_CACHE_DIR = Path(
+    os.environ.get("XDG_CACHE_HOME") or str(Path.home() / ".cache")
+) / "autoacct"
+_CACHE_PATH = _CACHE_DIR / "fx_cache.json"
+
+
+def _load_cache() -> dict:
+    if not _CACHE_PATH.exists():
+        return {}
+    try:
+        return json.loads(_CACHE_PATH.read_text())
+    except (json.JSONDecodeError, OSError):
+        return {}
+
+
+def _save_cache(cache: dict) -> None:
+    _CACHE_DIR.mkdir(parents=True, exist_ok=True)
+    tmp = _CACHE_PATH.with_suffix(".tmp")
+    tmp.write_text(json.dumps(cache, indent=2, sort_keys=True))
+    tmp.replace(_CACHE_PATH)
 
 
 def fetch_rate(currency: str, on_date: str) -> tuple[float, str]:
     currency = currency.upper()
     if currency == "HKD":
         return 1.0, on_date
+
+    cache = _load_cache()
+    key = f"{currency}_{on_date}"
+    if key in cache:
+        entry = cache[key]
+        return float(entry["rate"]), entry["fx_date"]
+
     url = f"https://api.frankfurter.dev/v1/{on_date}?from={currency}&to=HKD"
     req = urllib.request.Request(url, headers={"User-Agent": "AutoACCT/1.0"})
     with urllib.request.urlopen(req, timeout=10) as resp:
         data = json.loads(resp.read())
-    rate = data["rates"]["HKD"]
-    return float(rate), data["date"]
+    rate = float(data["rates"]["HKD"])
+    fx_date_resolved = data["date"]
+
+    cache[key] = {"rate": rate, "fx_date": fx_date_resolved}
+    _save_cache(cache)
+
+    return rate, fx_date_resolved
 
 
 def main() -> int: