feat: add domain entities (Clean Architecture core)

Pure domain types matching docs/data-model.md: User, Subscription, Video,
Transcript, Summary, plus Provider and TranscriptSource enums. Stdlib-only,
no outward dependencies — the innermost layer. Video carries user_id per the
per-user-isolation decision (no global dedup).

internal/domain/domain.go

@@ -0,0 +1,80 @@
+// Package domain holds Tapir's core entities. It depends on nothing outside
+// the standard library — no providers, no storage, no AI. See docs/data-model.md.
+package domain
+
+import "time"
+
+// Provider identifies a video platform.
+type Provider string
+
+const (
+	ProviderYouTube Provider = "youtube"
+	ProviderVimeo   Provider = "vimeo"
+)
+
+// TranscriptSource records how a transcript was obtained (or that there was none).
+type TranscriptSource string
+
+const (
+	SourceCaptions TranscriptSource = "captions"
+	SourceNone     TranscriptSource = "none"
+)
+
+// User is the Tapir-side profile. At Stage 0 there is exactly one.
+type User struct {
+	ID          string
+	DisplayName string
+	CreatedAt   time.Time
+}
+
+// Subscription is a watched channel on a connected account.
+type Subscription struct {
+	ID           string
+	UserID       string
+	ConnectionID string
+	ChannelID    string
+	ChannelTitle string
+	Active       bool
+}
+
+// Video is a single video seen for a user (per-user, not globally deduped —
+// see docs/data-model.md and DECISIONS.md "Rejected alternatives").
+type Video struct {
+	ID              string
+	UserID          string
+	SubscriptionID  string
+	Provider        Provider
+	ProviderVideoID string
+	Title           string
+	URL             string
+	PublishedAt     time.Time
+	SeenAt          time.Time
+}
+
+// Transcript is the text of a video (or a record that none was available).
+type Transcript struct {
+	VideoID  string
+	UserID   string
+	Source   TranscriptSource
+	Language string
+	Content  string // empty when Source == SourceNone
+}
+
+// HasText reports whether the transcript carries usable text.
+func (t Transcript) HasText() bool {
+	return t.Source != SourceNone && t.Content != ""
+}
+
+// Summary is the produced output for a video.
+type Summary struct {
+	ID           string
+	UserID       string
+	VideoID      string
+	Summary      string
+	Highlights   []string
+	Takeaways    []string
+	AIProvider   string // "local" | "anthropic" | "openai" | "gemini"
+	AIModel      string
+	FallbackUsed bool
+	CreatedAt    time.Time
+}