feat: add Linux support with cross-platform memory scanning

- Add Linux memory scanner (`find_all_keys_linux.py`) using `/proc/<pid>/mem`,
  same approach as Windows/macOS — no GDB, no function offsets, no restart needed
- Extract Windows-specific code to `find_all_keys_windows.py`
- Make `find_all_keys.py` a platform dispatcher (Windows / Linux)
- Add `key_utils.py` for cross-platform path matching (`/` vs `\` in all_keys.json)
- Update `config.py` with Linux auto-detection of db_storage paths
- Update all consumers (decrypt_db, monitor, monitor_web, mcp_server) to use
  `get_key_info()` for platform-agnostic key lookup

Tested on remote Linux container: 15/15 DBs scanned, decrypted, and verified.

key_utils.py

@@ -0,0 +1,29 @@
+import os
+
+
+def strip_key_metadata(keys):
+    """移除 all_keys.json 中以下划线开头的元数据字段。"""
+    return {k: v for k, v in keys.items() if not k.startswith("_")}
+
+
+def key_path_variants(rel_path):
+    """生成同一路径的多种分隔符表示，兼容 Windows/Linux JSON key。"""
+    normalized = rel_path.replace("\\", "/")
+    variants = []
+    for candidate in (
+        rel_path,
+        normalized,
+        normalized.replace("/", "\\"),
+        normalized.replace("/", os.sep),
+    ):
+        if candidate not in variants:
+            variants.append(candidate)
+    return variants
+
+
+def get_key_info(keys, rel_path):
+    """按相对路径查找数据库密钥，自动兼容不同平台分隔符。"""
+    for candidate in key_path_variants(rel_path):
+        if candidate in keys and not candidate.startswith("_"):
+            return keys[candidate]
+    return None