Remove cans feature; add Secure flag to pw_/tok_ cookies

Owner decision 2026-10-16: the cans feature (bundles of pastes, #4) was
half-integrated and has zero prod usage; removing it eliminates four audit
findings by deletion.

Removed:
- internal/api/cans.go and all can routes (/api/pastes/can, /api/cans/*,
  /can/{id}) plus handleCanPage/renderCan
- is_can from /api/mine and /api/public responses
- store layer: CanRow, can store funcs, paste_cans UNION in listings,
  cans sweep; reserved slugs 'can'/'cans' released
- templates/can.html, can CSS, is_can badges in history.js/mine.js
- cans tests (cans_test.go, cans_flow_test.go, cans_expiry_test.go,
  TestErrorCodeCanValidation)
- docs/api.md cans section, README/LICENSE/docker-compose mentions

Store migration: drops paste_cans only when empty (non-empty table is
kept and logged; prod verified clean 2026-10-16) and drops the dormant
pastes.can_id column. Covered by new TestMigrateDropsCansSchema.

Security hardening (audit findings): pw_<id> (unlock) and tok_<id>
(deletion-token) cookies now carry Secure: true in addition to
HttpOnly + SameSite=Lax; all deployments are HTTPS-only. cookie_set_test
now asserts Secure.

go vet ./... and go test ./... pass.

internal/store/customslug.go

@@ -10,7 +10,7 @@ var slugRE = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9-_]{0,63}$`)
 
 // reserved words that would collide with routes
 var reservedSlugs = map[string]bool{
-	"api": true, "raw": true, "can": true, "cans": true, "public": true,
+	"api": true, "raw": true, "public": true,
 	"history": true, "static": true, "assets": true, "favicon.ico": true,
 	"new": true, "login": true, "logout": true, "admin": true, "settings": true,
 	"mine": true, "saved": true, "unlock": true, "guess": true, "f": true,
@@ -36,12 +36,5 @@ func (s *Store) SlugTaken(slug string) (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	if n > 0 {
-		return true, nil
-	}
-	err = s.db.QueryRow(`SELECT COUNT(*) FROM paste_cans WHERE id = ?`, slug).Scan(&n)
-	if err != nil {
-		return false, err
-	}
 	return n > 0, nil
 }