Add image decryption and inline preview for WeChat V2 format

Support all three .dat encryption formats:
- Old XOR format: single-byte XOR, auto-detect key from magic bytes
- V1 format: AES-ECB with fixed key (md5("0")[:16]) + XOR tail
- V2 format (2025-08+): AES-128-ECB + raw middle + XOR tail

New files:
- decode_image.py: unified image decryption module (XOR/V1/V2)
- find_image_key.py: extract AES key from WeChat process memory
- find_image_key_monitor.py: continuous monitoring version for key capture

monitor_web.py changes:
- Inline image preview in Web UI with async decryption
- MonitorDBCache for mtime-based DB decryption caching
- username-to-DB mapping for image resolution chain
- /img/ endpoint for serving decoded images
- SSE image_update events for real-time preview updates

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

monitor.py

@@ -8,6 +8,9 @@ import hashlib, struct, os, sys, json, time, sqlite3, io
 import hmac as hmac_mod
 from datetime import datetime
 from Crypto.Cipher import AES
+import zstandard as zstd
+
+_zstd_dctx = zstd.ZstdDecompressor()
 
 sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-8', errors='replace')
 
@@ -218,6 +221,11 @@ def main():
 
                     # 消息内容
                     summary = curr['summary']
+                    if isinstance(summary, bytes):
+                        try:
+                            summary = _zstd_dctx.decompress(summary).decode('utf-8', errors='replace')
+                        except Exception:
+                            summary = '(压缩内容)'
                     if summary:
                         # 群消息格式: "wxid_xxx:\n内容" - 提取内容部分
                         if ':\n' in summary: