feat: one generic image, with installed modules in installation-owned storage (ADR 0048)

The production image now contains zero feature modules and is the same for every
installation. Installed modules live in module storage that one Core resolver
selects from EKKLESIA_MODULES_DIR: the checkout's modules/ when unset, an
absolute path that must exist when set (/var/lib/ekklesia/modules in the image).
A relative, blank, missing, or unreadable configured directory fails closed with
a sanitized error that never contains the value.

- Host configuration reads EKKLESIA_MODULES_DIR. The API, migration plan, apply
  and validate, module validation, module enablement, and Doctor all resolve it
  the same way; the test-only API override still takes precedence.
- Doctor receives Core's migrations directory explicitly instead of deriving it
  from the modules directory's parent.
- The migration runner retains and reports the applied history of a module that
  is not installed, and plans and applies Core's and present modules' migrations
  around it. Core's history and every installed module's history stay strict: a
  missing or changed applied migration, or a downgrade, still fails. No down
  migrations, no data deletion.
- deploy/Dockerfile copies no module and installs nothing for the module
  template; .dockerignore excludes modules/ and the build refuses a context that
  contains it; the runtime has an empty root-owned /var/lib/ekklesia/modules and
  sets EKKLESIA_MODULES_DIR.
- deploy/compose.yaml bind-mounts EKKLESIA_MODULES_HOST_DIR read-only into api
  and migrate, with create_host_path disabled.
- scripts/check-image.sh proves the image holds no modules and that a populated
  modules/ directory leaves its runtime file system identical; CI runs it.
- The split deploy smoke now uses the same generic image: it boots with no
  modules, installs a self-contained test module packaged from the template
  (test-only), migrates and restarts, removes it (Core runs, planning reports the
  retained history, data survives), and restores it with its data.

Docs: deploy README (module storage, install/update/remove/restore, the runtime
modules target), .env.example, API configuration, module development guide,
testing, workflow, security, and the architecture overview.

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

docs/architecture/platform-architecture.md

@@ -652,3 +652,23 @@ Still deliberately absent after Phase 8: a theme editor, a brand builder, or any
 module store or installer; a standalone host; signed write URLs, resumable uploads, or any body above
 the absolute ceiling; module-owned offline *content*; and every real feature module, Signage
 included. Nothing in this pass claims any of them exists.
+
+[ADR 0048](decisions/0048-generic-image-and-installed-module-storage.md) separates the platform
+artifact from what an installation installs:
+
+- **One generic image.** `deploy/Dockerfile` builds one image with zero feature modules, the same
+  for every installation; a checkout's `modules/` never reaches it.
+- **Module storage.** Installed modules live in installation-owned storage that one Core resolver
+  selects from `EKKLESIA_MODULES_DIR`. The API, every operator command, and Doctor use the same
+  resolver, and Core's own migrations are always found under the platform, never beside the
+  modules. A configured directory that is relative, blank, missing, or unreadable fails closed
+  with a sanitized error.
+- **Self-contained modules.** An installed module is a self-contained directory with its own
+  production dependencies; the platform lends it none. Discovery still happens only when a process
+  starts, so module changes take effect at a controlled restart and nothing is hot loaded.
+- **Removing keeps data.** Removing a module's code keeps its data and stored state. The migration
+  runner retains and reports the applied history of a module that is absent, instead of refusing
+  to plan, while Core's history and every installed module's history stay strict, so a returning
+  module that lost or changed an applied migration is still refused.
+- **Trust is unchanged.** Installed modules are still trusted code; module storage is mounted
+  read-only, which keeps Ekklesia from modifying installed code but does not sandbox it.