feat(youtube): VideoByID for arbitrary-video metadata (paste-a-URL)

videos.list (part=snippet) for a single id, including channels the user does
not follow. Data API call (1 quota unit), NOT the rate-limited caption path —
ungated metadata; only the later transcript fetch hits globalFetchGate. Returns
a subscription-less domain.Video scoped to the user, or ErrVideoNotFound for a
deleted/private/typo'd id. Foundation for Feature 2.

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

internal/adapters/youtube/youtube.go

@@ -18,6 +18,7 @@ package youtube
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -31,6 +32,10 @@ import (
 	"gitea.d-ma.be/mathias/tapir/internal/ports"
 )
 
+// ErrVideoNotFound is returned by VideoByID when an id resolves to no video
+// (deleted, private, or a typo'd paste). Callers map it to an honest user message.
+var ErrVideoNotFound = errors.New("youtube: video not found")
+
 // defaultBaseURL is the YouTube Data API v3 root. Overridable via Config.BaseURL
 // (tests point it at an httptest server).
 const defaultBaseURL = "https://www.googleapis.com/youtube/v3"
@@ -244,6 +249,37 @@ func (a *Adapter) NewVideos(ctx context.Context, sub domain.Subscription) ([]dom
 	return videos, nil
 }
 
+// VideoByID fetches a single video's metadata (videos.list, snippet) for an
+// arbitrary video id — including channels the user does not follow (paste-a-URL,
+// Feature 2). This is a Data API call (1 quota unit), NOT the rate-limited
+// caption path, so it is not gated: only the later transcript fetch goes through
+// globalFetchGate. UserID is set on the result and SubscriptionID is left empty
+// (a pasted video has no subscription parent). Returns ErrVideoNotFound when the
+// id resolves to no video.
+func (a *Adapter) VideoByID(ctx context.Context, userID, videoID string) (domain.Video, error) {
+	client, err := a.httpClient(ctx, a.cfg.TokenSecretRef)
+	if err != nil {
+		return domain.Video{}, err
+	}
+	q := url.Values{"part": {"snippet"}, "id": {videoID}}
+	var resp videoListResponse
+	if err := a.getJSON(ctx, client, "/videos", q, &resp); err != nil {
+		return domain.Video{}, fmt.Errorf("video by id %q: %w", videoID, err)
+	}
+	if len(resp.Items) == 0 {
+		return domain.Video{}, fmt.Errorf("video %q: %w", videoID, ErrVideoNotFound)
+	}
+	it := resp.Items[0]
+	return domain.Video{
+		UserID:          userID,
+		Provider:        domain.ProviderYouTube,
+		ProviderVideoID: videoID,
+		Title:           it.Snippet.Title,
+		URL:             "https://www.youtube.com/watch?v=" + videoID,
+		PublishedAt:     it.Snippet.PublishedAt,
+	}, nil
+}
+
 // uploadsPlaylistID derives a channel's uploads playlist id at zero API cost:
 // a standard channel id "UCxxxx" maps to uploads playlist "UUxxxx". Returns
 // ok=false for ids that don't follow this convention (caller falls back to
@@ -342,6 +378,15 @@ type playlistItemListResponse struct {
 	} `json:"items"`
 }
 
+type videoListResponse struct {
+	Items []struct {
+		Snippet struct {
+			Title       string    `json:"title"`
+			PublishedAt time.Time `json:"publishedAt"`
+		} `json:"snippet"`
+	} `json:"items"`
+}
+
 type channelListResponse struct {
 	Items []struct {
 		ContentDetails struct {