Phase 8: overrides (shared keycode + versioned admin lock/unlock/grant API)

Build on Phases 1–7. Adds the two override paths from AGENTS.md §4 FR-10/FR-11:
the shared device keycode and the versioned admin HTTP API the separate Kotlin
admin app will call.

- store: migration 004 adds state.grant_until and a settings table. New
  overrides.go records every override in `overrides` and verifies the shared
  keycode (bcrypt; one code for every device, §10 rule 3). ExpireLocks also
  clears an expired grant, and NextScheduleBoundary includes the grant end so a
  device re-locks on time.
- effective state: a grant unlocks a device inside a schedule window; a manual
  lock clears a grant and a grant clears a manual lock. The retained config
  payload gains grant_until (additive, v stays 1).
- web: /api/v1/admin/... (list, state, lock, unlock, grant) guarded by
  CONTROL_ADMIN_API_TOKEN, separate from the session login, plus the on-device
  keycode endpoint POST /api/v1/device/override with per-address throttling.
  A state change is published retained before the API responds.
- cmd/control: `keycode` subcommand and start-up seeding from
  CONTROL_DEVICE_KEYCODE_HASH.
- deploy/verify-phase8.sh + `make verify-phase8` prove the definition of done:
  an API lock reaches a real broker subscriber in well under a second.
- Version 0.6.0-phase8 (compose, Makefile, docs).

Measured on .13: the lock reached the TLS broker subscriber in 7 ms (handler
1 ms, request to retained publish accepted).

Tests: go vet ./... and go test -count=1 ./... pass in golang:1.25-alpine.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/mqtt/payload.go

@@ -86,9 +86,15 @@ type Config struct {
 	Allow    []string   `json:"allow"`
 	Block    []string   `json:"block"`
 	// Reason is why the device is locked: "schedule" for an active weekly
-	// window, "manual" for a lock from the dashboard. It is omitted when the
-	// device is not locked. It is additive (Phase 6), so v stays 1.
-	Reason      string    `json:"reason,omitempty"`
+	// window, "manual" for a lock from the dashboard or the admin API. It is
+	// omitted when the device is not locked. It is additive (Phase 6), so v
+	// stays 1.
+	Reason string `json:"reason,omitempty"`
+	// GrantUntil is set when a time-limited exemption is active (Phase 8): the
+	// device is unlocked and must not re-lock before this instant. It is omitted
+	// when there is no grant. Additive, so v stays 1.
+	GrantUntil *time.Time `json:"grant_until,omitempty"`
+
 	GeneratedAt time.Time `json:"generated_at"`
 }