Pair displays at the platform's own /display/setup, with or without modules

Displays are platform-owned, but a screen could only start pairing on a
module's display page: every pairing request named a module, so an
installation with no module that uses displays could not set one up.

- /display/setup is the platform's own display page, served whenever
  the deployment has device support, whatever is installed. It pairs a
  platform display with no module. Once paired, it opens the module it
  is set to show; with none set, it shows its name and waits, rechecking
  so it stays alive and notices revocation. "setup" is now a reserved
  module id.
- Migration core:0009_platform_display_pairing lets a pairing request
  name no module. It relaxes module_id and permission_id and adds a
  both-or-neither check. It is additive and destroys nothing.
- POST /api/v1/device-pairing/requests accepts {} for a platform
  display. Approval accepts an optional moduleId, so the administrator
  can choose an installed display module and the device gets that
  module's declared permission. A module display's own request can't be
  redirected.
- GET /api/v1/device-pairing/self lets a paired device describe itself:
  its name and the module displays it may show. It renews the device's
  cookie.
- GET /api/v1/devices/setup is replaced by
  GET /api/v1/devices/display-modules: the modules a display can be set
  to show.
- Settings -> Displays always offers the /display/setup address, Copy
  setup link and Add display. Add display has an optional "Show on this
  screen" choice, and each paired display says what it shows.
- /display/<module-id> is unchanged, so released Signage 0.1.5 works as
  before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

core/drizzle/0009_platform_display_pairing.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "ekklesia_core_device_pairing_request" ALTER COLUMN "module_id" DROP NOT NULL;--> statement-breakpoint
+ALTER TABLE "ekklesia_core_device_pairing_request" ALTER COLUMN "permission_id" DROP NOT NULL;--> statement-breakpoint
+ALTER TABLE "ekklesia_core_device_pairing_request" ADD CONSTRAINT "ekklesia_core_device_pairing_module_check" CHECK (("ekklesia_core_device_pairing_request"."module_id" is null) = ("ekklesia_core_device_pairing_request"."permission_id" is null));

core/drizzle/meta/_ekklesia.json

@@ -55,6 +55,12 @@
       "destructive": {
         "mode": "none"
       }
+    },
+    {
+      "tag": "0009_platform_display_pairing",
+      "destructive": {
+        "mode": "none"
+      }
     }
   ]
 }

core/drizzle/meta/_journal.json

@@ -64,6 +64,13 @@
       "when": 1792036365485,
       "tag": "0008_user_preferences",
       "breakpoints": true
+    },
+    {
+      "idx": 9,
+      "version": "7",
+      "when": 1792124723290,
+      "tag": "0009_platform_display_pairing",
+      "breakpoints": true
     }
   ]
 }