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]>

README.md

@@ -35,6 +35,18 @@ concurrency and release, exact destructive approvals, and sanitization against t
 adapter. A Core-owned `pnpm doctor` dashboard combines the existing static discovery and read-only
 migration-planning services into one concise readiness report.
 
+**Ekklesia ships one generic image, and installed modules live outside it
+([ADR 0048](docs/architecture/decisions/0048-generic-image-and-installed-module-storage.md)).** The
+production image built from [`deploy/Dockerfile`](deploy/Dockerfile) contains zero feature modules
+and is the same for every installation. Installed modules live in installation-owned module storage,
+selected by `EKKLESIA_MODULES_DIR` (`/var/lib/ekklesia/modules` in the image, the checkout's
+`modules/` when unset) and mounted read-only by the reference deployment. Each installed module is a
+self-contained directory carrying its own production dependencies. Installing, updating, or removing
+a module needs only a controlled restart and, when it has migrations, an explicit plan and apply.
+Nothing is hot loaded. Removing a module's code keeps its data and its stored state; its applied
+migration history is retained and reported, and blocks no other migration. Purge, a package format,
+signing, and any installer or marketplace do not exist yet.
+
 Phase 4A added Core-owned identity foundations: a Core-owned Better Auth database schema
 (`ekklesia_core_user`, `ekklesia_core_session`, `ekklesia_core_account`,
 `ekklesia_core_verification`), a first-class Core-owned migration source that coexists with