feat(uploads): validation policy for probed audio uploads (ADR-033)

Video is checked first so it is always refused as video; limits and reasons
are typed so callers and tests branch on Reason, not message text.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/uploads/validate.go

@@ -0,0 +1,82 @@
+// Package uploads is the use case behind "upload an audio recording" (ADR-033):
+// validate the file by PROBING it, keep the transcript private, never retain the
+// audio. It depends only on ports (a Prober, a store, a job Submitter) — the
+// concrete ffprobe adapter, the web handler and the transcription worker are wired
+// from outside.
+package uploads
+
+import (
+	"fmt"
+	"time"
+)
+
+// Limits bound what an upload may be. Both are rejected synchronously, with a
+// specific message, rather than by a timeout (ADR-033).
+type Limits struct {
+	MaxBytes    int64
+	MaxDuration time.Duration
+}
+
+// Probe is what inspecting an uploaded file revealed. It is the input to Validate
+// because a name or MIME type cannot say whether a container carries video: a
+// Teams voice-only export is typically an .mp4 with no video stream.
+type Probe struct {
+	Duration time.Duration
+	HasAudio bool
+	HasVideo bool
+}
+
+// Reason is the machine-checkable class of a rejection.
+type Reason string
+
+const (
+	ReasonVideo      Reason = "video"
+	ReasonNoAudio    Reason = "no_audio"
+	ReasonTooLong    Reason = "too_long"
+	ReasonTooLarge   Reason = "too_large"
+	ReasonUnreadable Reason = "unreadable"
+)
+
+// RejectError is a synchronous, user-presentable refusal. Message is written for
+// the person who uploaded the file; Reason is for code and tests.
+type RejectError struct {
+	Reason  Reason
+	Message string
+}
+
+func (e *RejectError) Error() string { return string(e.Reason) + ": " + e.Message }
+
+// Validate decides whether a probed file may be transcribed. Video is checked
+// first so a video file is always refused as video, whatever its length.
+func Validate(p Probe, l Limits) error {
+	switch {
+	case p.HasVideo:
+		return &RejectError{Reason: ReasonVideo, Message: "This file contains a video track. " +
+			"Only audio recordings are supported for now; video upload is tracked in issue #28."}
+	case !p.HasAudio:
+		return &RejectError{Reason: ReasonNoAudio, Message: "This file has no audio track to transcribe."}
+	case p.Duration <= 0:
+		return &RejectError{Reason: ReasonUnreadable, Message: "Could not read a length from this file; it may be damaged."}
+	case p.Duration > l.MaxDuration:
+		return &RejectError{Reason: ReasonTooLong, Message: fmt.Sprintf(
+			"This recording is %s long; the limit is %s.", FormatDuration(p.Duration), FormatDuration(l.MaxDuration))}
+	}
+	return nil
+}
+
+// FormatDuration renders a length for people: "1 h 42 min", "45 min", "30 s".
+func FormatDuration(d time.Duration) string {
+	d = d.Round(time.Second)
+	h := int(d / time.Hour)
+	m := int(d % time.Hour / time.Minute)
+	switch {
+	case h > 0 && m > 0:
+		return fmt.Sprintf("%d h %d min", h, m)
+	case h > 0:
+		return fmt.Sprintf("%d h", h)
+	case m > 0:
+		return fmt.Sprintf("%d min", m)
+	default:
+		return fmt.Sprintf("%d s", int(d/time.Second))
+	}
+}