#38 iteration 1: file attachments, 1 file per paste

- internal/store/blob.go: BlobStore interface + fs implementation with
  traversal-safe keys (<paste-id>/<sha256>), put/get/stat/delete
- attachments table migration (id, paste_id, filename sanitized to 255,
  mime sniffed server-side, size, sha256, created_at)
- POST /api/pastes now accepts multipart/form-data with a 'file' part;
  1 file = 1 paste: file replaces text content when both are sent
- 25 MB per-file limit enforced server-side (413 file_too_large)
- GET /f/{attachment-id}/{filename}: stored sniffed mime, nosniff,
  inline only for images/pdf, html/svg/xml forced to text/plain (#34 rule)
- paste view renders attachment chip + inline image preview
- /new: dropzone with file picker, drag-and-drop, Ctrl+V file paste,
  file chip with name/size/remove, matches pill/radius design
- tests: blob roundtrip/traversal/sanitize; multipart create (mime
  sniffing, client mime ignored, size limit, two-file reject, html/svg
  forcing, 404s, password/expiry fields)

internal/store/attachment.go

@@ -0,0 +1,147 @@
+package store
+
+import (
+	"crypto/sha256"
+	"database/sql"
+	"errors"
+	"fmt"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// #38: file attachments, one file per paste (iteration 1). A paste either
+// has text content OR one attached file. Bytes live in the BlobStore; this
+// table carries the metadata.
+
+type Attachment struct {
+	ID        string `json:"id"`
+	PasteID   string `json:"paste_id"`
+	Filename  string `json:"filename"`
+	Mime      string `json:"mime"`
+	Size      int64  `json:"size"`
+	SHA256    string `json:"sha256"`
+	CreatedAt int64  `json:"created_at"`
+
+	SizeHuman string `json:"-"` // template-only: human-readable size
+}
+
+const MaxFilenameLen = 255
+
+// ErrFileTooLarge is returned when an attachment exceeds the per-file cap.
+var ErrFileTooLarge = errors.New("file too large")
+
+// SanitizeFilename cleans a user-supplied filename: strips directory
+// components, control chars, and caps at 255 bytes. The extension is never
+// trusted for mime decisions (mime is sniffed server-side).
+func SanitizeFilename(name string) string {
+	name = filepath.Base(strings.ReplaceAll(name, "\\", "/"))
+	name = strings.TrimSpace(name)
+	var b strings.Builder
+	for _, r := range name {
+		if r < 32 || r == 127 {
+			continue
+		}
+		b.WriteRune(r)
+	}
+	name = b.String()
+	if len(name) > MaxFilenameLen {
+		runes := []rune(name)
+		for len(string(runes)) > MaxFilenameLen {
+			runes = runes[:len(runes)-1]
+		}
+		name = string(runes)
+	}
+	if name == "" || name == "." || name == ".." {
+		name = "file"
+	}
+	return name
+}
+
+// CreateAttachment stores the file bytes and inserts the attachments row.
+// The mime MUST already be sniffed server-side (http.DetectContentType by
+// the caller); it is never taken from the client.
+func (s *Store) CreateAttachment(a *Attachment, r interface{ Read([]byte) (int, error) }, blobs BlobStore) error {
+	pasteID := a.PasteID
+	sha, size, err := blobs.Put(pasteID+"/pending", r)
+	if err != nil {
+		return err
+	}
+	if a.Size > 0 && size > a.Size {
+		// caller-provided pre-check limit; treat as too large
+		blobs.Delete(pasteID + "/" + sha)
+		return ErrFileTooLarge
+	}
+	a.SHA256 = sha
+	a.Size = size
+	now := time.Now().Unix()
+	id := genSlug(20)
+	_, err = s.db.Exec(`INSERT INTO attachments (id, paste_id, filename, mime, size, sha256, created_at)
+		VALUES (?,?,?,?,?,?,?)`, id, pasteID, a.Filename, a.Mime, size, sha, now)
+	if err != nil {
+		blobs.Delete(pasteID + "/" + sha)
+		return err
+	}
+	a.ID = id
+	a.CreatedAt = now
+	return nil
+}
+
+// GetAttachment returns the attachment row for a paste, or nil.
+// One file per paste (iteration 1), so a single-row lookup keyed on paste.
+func (s *Store) GetAttachmentForPaste(pasteID string) (*Attachment, error) {
+	row := s.db.QueryRow(`SELECT id, paste_id, filename, mime, size, sha256, created_at
+		FROM attachments WHERE paste_id = ? ORDER BY created_at ASC LIMIT 1`, pasteID)
+	return scanAttachment(row)
+}
+
+func (s *Store) GetAttachment(id string) (*Attachment, error) {
+	row := s.db.QueryRow(`SELECT id, paste_id, filename, mime, size, sha256, created_at
+		FROM attachments WHERE id = ?`, id)
+	return scanAttachment(row)
+}
+
+func scanAttachment(row *sql.Row) (*Attachment, error) {
+	var a Attachment
+	err := row.Scan(&a.ID, &a.PasteID, &a.Filename, &a.Mime, &a.Size, &a.SHA256, &a.CreatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	a.SizeHuman = humanBytes(a.Size)
+	return &a, nil
+}
+
+// humanBytes formats a byte count for display (KB/MB).
+func humanBytes(n int64) string {
+	switch {
+	case n >= 1<<20:
+		return fmt.Sprintf("%.1f MB", float64(n)/(1<<20))
+	case n >= 1<<10:
+		return fmt.Sprintf("%.1f KB", float64(n)/(1<<10))
+	default:
+		return fmt.Sprintf("%d B", n)
+	}
+}
+
+// DeleteAttachment removes the row and its blob.
+func (s *Store) DeleteAttachment(a *Attachment, blobs BlobStore) error {
+	blobs.Delete(a.PasteID + "/" + a.SHA256)
+	_, err := s.db.Exec(`DELETE FROM attachments WHERE id = ?`, a.ID)
+	return err
+}
+
+// HasAttachment reports whether a paste already carries a file (#38: one file per paste).
+func (s *Store) HasAttachment(pasteID string) bool {
+	var n int
+	s.db.QueryRow(`SELECT COUNT(*) FROM attachments WHERE paste_id = ?`, pasteID).Scan(&n)
+	return n > 0
+}
+
+// HashBytes is a small helper used by handlers to name/verify blobs.
+func HashBytes(b []byte) string {
+	h := sha256.Sum256(b)
+	return string(h[:])
+}