Support multiple databases or directory scanning

[ojongerius]

Context

Different tools write receipt databases to different locations:

• openclaw: ~/.openclaw/agent-receipts/receipts.db
• mcp-proxy: user-configured path
• SDK tests: arbitrary temp paths

The dashboard currently accepts a single --db flag and opens one database at a time.

What needs to change

• Support multiple --db flags to open several databases simultaneously
• Consider a --dir flag that scans a directory for .db files
• Add a database switcher or multi-database view in the UI
• Keep databases visually distinct (labeled by path or source)

Why

Users running multiple agents or both the openclaw plugin and mcp-proxy will have receipts spread across databases. Switching between separate dashboard instances is cumbersome.