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/api/cookie_set_test.go

@@ -1,7 +1,7 @@
 package api
 
-// #143: create responses must set the short-lived tok_<id> HttpOnly cookie
-// that the paste view reads for the one-time created banner.
+// #143: create responses must set the short-lived tok_<id> HttpOnly+Secure
+// cookie that the paste view reads for the one-time created banner.
 
 import (
 	"net/http"
@@ -29,6 +29,9 @@ func TestCreateSetsDeletionTokenCookie(t *testing.T) {
 			if !c.HttpOnly {
 				t.Error("tok_ cookie not HttpOnly")
 			}
+			if !c.Secure {
+				t.Error("tok_ cookie not Secure")
+			}
 			if c.MaxAge != 60 {
 				t.Errorf("tok_ cookie MaxAge = %d, want 60", c.MaxAge)
 			}