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/web/processing.go

@@ -3,6 +3,8 @@ package web
 import (
 	"context"
 	"sync"
+
+	"gitea.d-ma.be/mathias/tapir/internal/domain"
 )
 
 // Processor runs the core summarization use case for a single already-discovered
@@ -14,6 +16,14 @@ type Processor interface {
 	ProcessVideo(ctx context.Context, userID, videoID string) error
 }
 
+// VideoFetcher resolves an arbitrary YouTube video id to its metadata for the
+// paste-a-URL flow (Feature 2). It is a Data API call, NOT the rate-limited
+// caption path. Returns domain.ErrVideoNotFound for a deleted/private/typo'd id.
+// cmd/tapir wires a per-user YouTube adapter; nil disables the paste route.
+type VideoFetcher interface {
+	FetchVideo(ctx context.Context, userID, videoID string) (domain.Video, error)
+}
+
 // ProcessingSet tracks the (user, video) ids currently being summarized in-process
 // so the status endpoint can show the animation until the summary lands. It is
 // ephemeral (single-instance Stage-1): a restart drops it, and the DB holds the