feat(web): paste-a-URL handler — add an arbitrary video + summarize (Feature 2)

POST /paste: parse the video id, fetch metadata via the VideoFetcher port (Data
API, ungated), upsert a subscription-less row scoped to the user (idempotent =
dedup), and — unless already summarized — RequestSummarize + start immediate
processing through the SAME globalFetchGate as the Summarize button. Explicit
paste overrides the recency window; a captionless video degrades to the honest
'no transcript' terminal state via the engine (ADR-010). Invalid URL -> 400,
not-found -> 404, both add nothing. Route mounts only when a Fetcher is wired.

Moves the video-not-found sentinel to domain (shared by adapter + web, no
cross-adapter coupling). Tests: valid add+queue, invalid, not-found, dedup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/domain/domain.go

@@ -3,10 +3,16 @@
 package domain
 
 import (
+	"errors"
 	"fmt"
 	"time"
 )
 
+// ErrVideoNotFound is returned when a video id resolves to no video (deleted,
+// private, or a typo'd paste). Defined in domain so adapters and the web layer
+// share one sentinel without coupling to each other.
+var ErrVideoNotFound = errors.New("video not found")
+
 // ErrChannelUnavailable is returned by a VideoSource when a channel's upload
 // playlist returns HTTP 404 — the channel was deleted or made private. The runner
 // stores these so the account page can surface them to the user.