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]>

deploy/compose.yaml

@@ -32,8 +32,20 @@ x-ekklesia-app: &ekklesia-app
     # without any storage TLS exception. The database CA travels separately, as PEM text in
     # EKKLESIA_DATABASE_CA, and is the only CA the database connection trusts.
     NODE_EXTRA_CA_CERTS: /run/ekklesia/tls/ca.crt
+    # Installed modules (ADR 0048). The image sets the same value; stating it beside the mount below
+    # keeps the variable and the mount target from ever drifting apart.
+    EKKLESIA_MODULES_DIR: /var/lib/ekklesia/modules
   volumes:
     - ${EKKLESIA_TLS_DIR:-/etc/ekklesia/tls}/ca.crt:/run/ekklesia/tls/ca.crt:ro
+    # Installation-owned module storage, outside the image (ADR 0048). Read-only: Ekklesia never
+    # writes installed code. The host directory must already exist; a missing one stops the
+    # container from starting instead of Compose quietly creating an empty, root-owned directory.
+    - type: bind
+      source: ${EKKLESIA_MODULES_HOST_DIR:-/var/lib/ekklesia/modules}
+      target: /var/lib/ekklesia/modules
+      read_only: true
+      bind:
+        create_host_path: false
   networks:
     data: {}
     storage: {}