Formatted feed discovery paths return 500 for malformed encoding

[phucnguyen1707]

Formatted feed discovery paths currently return HTTP 500 when the keyword segment contains malformed percent-encoding.

This affects routes like:

• /rss/discover/:keyword.xml
• /opml/discover/:keyword.xml
• /atom/discover/:keyword.xml
• /json-feed/discover/:keyword.json

Repro:

curl -i 'http://localhost:4010/rss/discover/%E0%A4%A.xml'

Current behavior:

{ "error": "URI malformed" }

Expected behavior:

{ "error": "Invalid path encoding" }

This should be treated as a 400 client input error, not an internal server error.