Add CSV export of scan results

The Review tab gets an Export CSV menu: "This view" exports the current
status filter and search, "Entire scan" exports every scanned slave file.

Each row carries status, match type, a recommendation, slave/master paths,
the link target, the match reason, sizes, codec details for both sides,
the quality verdict, notes, errors and timestamps. Rows are streamed in
pages of 1000 so a large export never holds the database connection a
running scan needs. The file starts with a UTF-8 BOM for Excel, and cells
a spreadsheet would evaluate as formulas are prefixed with an apostrophe.

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

internal/api/export.go

@@ -0,0 +1,179 @@
+package api
+
+import (
+	"encoding/csv"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/cdnutter/playlink/internal/engine"
+	"github.com/cdnutter/playlink/internal/probe"
+	"github.com/cdnutter/playlink/internal/store"
+)
+
+var kindLabels = map[string]string{
+	store.KindPath:      "same path",
+	store.KindPathCase:  "path differs in case",
+	store.KindExtension: "other format",
+	store.KindFuzzy:     "similar title",
+	store.KindElsewhere: "same top folder, other subfolder",
+	store.KindApprox:    "approximate match",
+	store.KindNone:      "no match",
+}
+
+var exportHeader = []string{
+	"status", "match_type", "ignored", "recommendation",
+	"slave_path", "master_path", "link_target", "reason",
+	"slave_size_bytes", "master_size_bytes",
+	"slave_codec", "slave_lossless", "slave_bitrate_kbps", "slave_sample_rate_hz", "slave_bit_depth", "slave_duration_s",
+	"master_codec", "master_lossless", "master_bitrate_kbps", "master_sample_rate_hz", "master_bit_depth", "master_duration_s",
+	"better_copy", "duration_difference_s", "notes", "error",
+	"slave_modified", "master_modified", "checked_at",
+}
+
+// csvSafe neutralises cells a spreadsheet would evaluate as a formula (a file
+// named "=HYPERLINK(...).mp3" must stay text).
+func csvSafe(s string) string {
+	if s != "" && strings.ContainsRune("=+-@\t\r", rune(s[0])) {
+		return "'" + s
+	}
+	return s
+}
+
+func recommendation(m store.Match, v probe.Verdict) string {
+	switch {
+	case m.Ignored:
+		return "ignored"
+	case m.Status == store.StatusLinked:
+		return "done"
+	case m.Status == store.StatusIdentical:
+		return "link (identical)"
+	case m.Status == store.StatusDifferent || m.Status == store.StatusSimilar:
+		switch v.Better {
+		case "master":
+			return "upgrade to master copy"
+		case "slave":
+			return "review: master is lower quality"
+		}
+		return "review"
+	case m.Status == store.StatusError:
+		return "fix error"
+	}
+	return ""
+}
+
+func probeCols(raw json.RawMessage) []string {
+	if len(raw) == 0 {
+		return make([]string, 6)
+	}
+	var p probe.Info
+	if json.Unmarshal(raw, &p) != nil || p.Error != "" {
+		return make([]string, 6)
+	}
+	num := func(n int) string {
+		if n == 0 {
+			return ""
+		}
+		return strconv.Itoa(n)
+	}
+	bitrate := ""
+	if p.Bitrate > 0 {
+		bitrate = strconv.FormatInt((p.Bitrate+500)/1000, 10)
+	}
+	dur := ""
+	if p.Duration > 0 {
+		dur = strconv.FormatFloat(p.Duration, 'f', 1, 64)
+	}
+	return []string{p.Codec, strconv.FormatBool(p.Lossless), bitrate, num(p.SampleRate), num(p.BitDepth), dur}
+}
+
+func nsTime(ns int64) string {
+	if ns == 0 {
+		return ""
+	}
+	return time.Unix(0, ns).UTC().Format(time.RFC3339)
+}
+
+var unsafeName = regexp.MustCompile(`[^A-Za-z0-9._-]+`)
+
+// handleExport streams the pair's scan results as CSV. It honours the same
+// status and search filters as the Review list.
+func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	p, err := s.st.Pair(r.Context(), id)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	q := r.URL.Query()
+	f := store.MatchFilter{Status: q.Get("status"), Query: q.Get("q")}
+
+	scope := f.Status
+	if scope == "" {
+		scope = "all"
+	}
+	slug := strings.Trim(unsafeName.ReplaceAllString(p.Name, "-"), "-")
+	if slug == "" {
+		slug = fmt.Sprintf("pair%d", p.ID)
+	}
+	name := fmt.Sprintf("playlink-%s-%s-%s.csv", slug, scope, time.Now().Format("20060102-1504"))
+	w.Header().Set("Content-Type", "text/csv; charset=utf-8")
+	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"; filename*=UTF-8''%s`, name, url.PathEscape(name)))
+
+	// A UTF-8 byte order mark makes Excel read non-ASCII names correctly.
+	_, _ = w.Write([]byte{0xEF, 0xBB, 0xBF})
+	cw := csv.NewWriter(w)
+	_ = cw.Write(exportHeader)
+	rows := 0
+	err = s.st.ExportMatches(r.Context(), id, f, func(m store.Match) error {
+		var v probe.Verdict
+		_ = json.Unmarshal(m.Verdict, &v)
+		target := ""
+		if m.RelMaster != "" && m.Status != store.StatusLinked {
+			target = engine.LinkTarget(m.RelSlave, m.RelMaster, false)
+		}
+		size := func(n int64) string {
+			if n == 0 {
+				return ""
+			}
+			return strconv.FormatInt(n, 10)
+		}
+		delta := ""
+		if v.DurationDelta > 0 {
+			delta = strconv.FormatFloat(v.DurationDelta, 'f', 1, 64)
+		}
+		rec := []string{
+			m.Status, kindLabels[m.Kind], strconv.FormatBool(m.Ignored), recommendation(m, v),
+			csvSafe(m.RelSlave), csvSafe(m.RelMaster), csvSafe(target), csvSafe(m.Reason),
+			size(m.SlaveSize), size(m.MasterSize),
+		}
+		rec = append(rec, probeCols(m.SlaveProbe)...)
+		rec = append(rec, probeCols(m.MasterProbe)...)
+		better := v.Better
+		if better == "unknown" {
+			better = ""
+		}
+		rec = append(rec, better, delta, csvSafe(strings.Join(v.Notes, "; ")), csvSafe(m.Note),
+			nsTime(m.SlaveMtime), nsTime(m.MasterMtime), time.Unix(m.UpdatedAt, 0).UTC().Format(time.RFC3339))
+		rows++
+		if err := cw.Write(rec); err != nil {
+			return err
+		}
+		if rows%500 == 0 {
+			cw.Flush()
+		}
+		return cw.Error()
+	})
+	cw.Flush()
+	if err != nil {
+		// Headers are already sent; the truncated file is the only signal left.
+		s.log.Warn("csv export interrupted", "pair", id, "rows", rows, "err", err)
+		return
+	}
+	s.log.Info("csv export", "pair", p.Name, "filter", scope, "rows", rows)
+}