docs(collection): document all roles and fix metadata drift

Replace ansible-galaxy init placeholders across the collection and
correct documentation that drifted from the code, after a multi-agent
review of every role README against its defaults, tasks and templates.

Collection level:
- README: role table for all 16 roles, requirements and role-ordering
- galaxy.yml: declare community.docker and community.general deps,
  real description/tags/urls; normalize license to MIT-0
- meta/runtime.yml: requires_ansible '>=2.15.0'
- plugins/README: document the homarr_layout filter and
  garage_credentials lookup instead of scaffold boilerplate

Per-role meta/main.yml and README for the placeholder roles
(389ds, authentik, authentik_outpost_ldap, base, collabora, drawio,
garage, homarr, httpbin, keycloak, nextcloud, opencloud, traefik).

Correctness fixes found during review:
- keycloak: wrong domain default, drop invented keycloak_cert_resolver,
  document the provisioning feature
- garage: root_domain is .s3.<first-entry>, not the bare domain
- opnform: jwt/front_api secrets use `openssl rand -hex 32`; align the
  validation fail_msg in tasks/main.yml accordingly
- send: S3 example references garage_s3_domains[0] (was singular)
- opencloud: document required opencloud_wopi_domain

License normalized to MIT-0 across galaxy.yml, role meta and READMEs to
match the SPDX headers.

galaxy.yml

@@ -23,12 +23,12 @@ authors:
 
 ### OPTIONAL but strongly recommended
 # A short summary description of the collection
-description: your collection description
+description: Ansible roles to deploy the Digitalboard self-hosted service platform (Docker Compose + Traefik + SSO)
 
 # Either a single license or a list of licenses for content inside of a collection. Ansible Galaxy currently only
 # accepts L(SPDX,https://spdx.org/licenses/) licenses. This key is mutually exclusive with 'license_file'
 license:
-- GPL-2.0-or-later
+- MIT-0
 
 # The path to the license file for the collection. This path is relative to the root of the collection. This key is
 # mutually exclusive with 'license'
@@ -36,25 +36,36 @@ license_file: ''
 
 # A list of tags you want to associate with the collection for indexing/searching. A tag name has the same character
 # requirements as 'namespace' and 'name'
-tags: []
+tags:
+  - digitalboard
+  - docker
+  - traefik
+  - sso
+  - selfhosted
 
 # Collections that this collection requires to be installed for it to be usable. The key of the dict is the
 # collection label 'namespace.name'. The value is a version range
 # L(specifiers,https://python-semanticversion.readthedocs.io/en/latest/#requirement-specification). Multiple version
 # range specifiers can be set and are separated by ','
-dependencies: {}
+dependencies:
+  # Used by nearly every role: docker_compose_v2, docker_container,
+  # docker_container_exec, docker_network. Hard runtime dependency.
+  community.docker: '>=3.0.0'
+  # Used by the keycloak role (keycloak_realm/client/group/user and
+  # related modules) in roles/keycloak/tasks/provisioning.yml.
+  community.general: '>=7.0.0'
 
 # The URL of the originating SCM repository
 repository: https://git.digitalboard.ch/Digitalboard/digitalboard.core
 
 # The URL to any online docs
-documentation: http://docs.example.com
+documentation: https://git.digitalboard.ch/Digitalboard/digitalboard.core
 
 # The URL to the homepage of the collection/project
-homepage: http://example.com
+homepage: https://git.digitalboard.ch/Digitalboard/digitalboard.core
 
 # The URL to the collection issue tracker
-issues: http://example.com/issue/tracker
+issues: https://git.digitalboard.ch/Digitalboard/digitalboard.core/issues
 
 # A list of file glob-like patterns used to filter any files or directories that should not be included in the build
 # artifact. A pattern is matched from the relative path of the file or directory of the collection directory. This