add top-level AT-URI resolution route (#1206)
support URLs like https://plyr.fm/at://did:plc:xxx/fm.plyr.track/rkey
by resolving them via backend by-uri endpoints and 301 redirecting
to the canonical page (/track/{id}, /playlist/{id}).
uses AtUri from @atproto/api for proper URI parsing and validation.
ref: streamplace/streamplace#1012
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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