Adopt shared-SA model, add DEPLOY guide, MIT license

- Install flow now assumes one admin-distributed service account JSON shared
  across the team. Each user creates their own Google Sheet and shares it
  with the SA email; per-user GCP project setup is gone.
- DEPLOY.md: a step-by-step walkthrough for non-technical users following
  along with an AI agent (Terminal basics, screenshots-worth of expected
  output, common-error table).
- scripts/append_row.py: sheet_id field accepts either a bare ID or a full
  Google Sheets URL; normalize_sheet_id() extracts the ID via regex.
- scripts/setup.md: rewritten as an admin guide (one-time GCP setup, key
  rotation) plus a troubleshooting reference.
- LICENSE: MIT (previously "private — internal use"). README license
  sections updated to match.

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

scripts/append_row.py

@@ -16,6 +16,7 @@ from __future__ import annotations
 
 import json
 import os
+import re
 import sys
 from datetime import datetime, timezone
 from pathlib import Path
@@ -26,6 +27,13 @@ from googleapiclient.discovery import build
 
 CONFIG_PATH = Path(__file__).resolve().parent.parent / "config.json"
 
+
+def normalize_sheet_id(value: str) -> str:
+    # Accept either a bare sheet ID or a full Google Sheets URL —
+    # e.g. https://docs.google.com/spreadsheets/d/<ID>/edit#gid=0.
+    m = re.search(r"/d/([a-zA-Z0-9_-]+)", value.strip())
+    return m.group(1) if m else value.strip()
+
 COLUMNS = [
     "date", "merchant", "category", "amount", "currency",
     "amount_hkd", "fx_rate", "fx_date", "payment_method",
@@ -63,7 +71,7 @@ def main() -> int:
         svc.spreadsheets()
         .values()
         .append(
-            spreadsheetId=cfg["sheet_id"],
+            spreadsheetId=normalize_sheet_id(cfg["sheet_id"]),
             range=f"{cfg['worksheet']}!A1",
             valueInputOption="USER_ENTERED",
             insertDataOption="INSERT_ROWS",