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/web/static/mine.js

@@ -18,8 +18,8 @@ const t = PaletteTable.init({
   rowHtml: it =>
     `<tr class="row" data-href="/${t.esc(it.id)}"><td>` +
     (it.title
-      ? `<a class="paste-name" href="/${t.esc(it.id)}">${t.esc(it.title)}</a>${it.is_can ? ' <span class="badge" title="Can — bundle of items">can</span>' : ''}`
-      : `<a class="slug paste-name" href="/${t.esc(it.id)}">${t.esc(it.id)}</a>${it.is_can ? ' <span class="badge" title="Can — bundle of items">can</span>' : ''}`) +
+      ? `<a class="paste-name" href="/${t.esc(it.id)}">${t.esc(it.title)}</a>`
+      : `<a class="slug paste-name" href="/${t.esc(it.id)}">${t.esc(it.id)}</a>`) +
     `</td>` +
     `<td><span class="badge">${t.esc(it.type || it.language || 'text')}</span></td>` +
     `<td class="dim">${t.fmtSize(it.size)}</td><td class="dim" data-ts="${it.created_at}">${t.ago(it.created_at)}</td>` +