feat: add host-provided module object storage

`manifest.storage.buckets` has existed since manifest version 1 and meant
nothing — discovery validated it and no code read it. This turns it into a
real capability, which is what stands between Announcements and the module
system: that product currently fetches a hardcoded LAN S3 endpoint, parses
ListObjectsV2 XML by hand, and hands the browser direct object URLs.

- A module declares a logical bucket and receives a scoped facade at
  `context.storage` — list, head, get, put, delete. The contract names no
  provider: no endpoint, region, credential, or physical bucket is reachable
  from it, which is what lets the same module run under a future standalone
  host over its own or a shared S3-compatible service.
- Core's first provider speaks S3 through the AWS SDK. SeaweedFS is the
  deployment target (ADR 0004), never an API name. Probing the pinned image
  found `CreateBucket` reporting `BucketAlreadyExists` rather than AWS's
  `BucketAlreadyOwnedByYou`; treating only the AWS spelling as success would
  have failed every second boot. A live test pins it.
- Logical → physical mapping is deterministic and collision-safe: an
  installation prefix, the module and bucket ids joined by `--` (which cannot
  occur inside either identifier, so `a-b`+`c` and `a`+`b-c` cannot collide),
  and a digest over the full untruncated identity. Two modules declaring
  `assets` always get separate namespaces; neither can see the physical name.
- Declaring a bucket is now a requirement: with no provider configured the
  module is `blocked · storage_unavailable` through ADR 0028's existing
  resolver, propagating to required dependents. An installation with no object
  storage stays a complete deployment.
- Binding availability is not provider health. Eligibility asks whether
  storage is configured, never whether it answered; bucket creation is lazy on
  first use so a storage outage cannot stop the platform booting; a running
  module whose store blinks stays running and retries. Nothing deletes a
  bucket.
- Credentials are deployment-supplied, never persisted, never routed through
  module configuration, and never visible to modules, module UI, diagnostics,
  or API responses. Provider errors translate to a closed code set carrying no
  endpoint, bucket, request id, or XML.
- Retires ADR 0029's transitional `bucket` configuration key. No manifest
  version change. The template declares no bucket, so it still runs without S3.

See ADR 0030.

Co-Authored-By: Claude Opus 5 <[email protected]>

.env.example

@@ -12,10 +12,22 @@ POSTGRES_PASSWORD=ekklesia-development-only
 POSTGRES_PORT=5432
 DATABASE_URL=postgresql://ekklesia:[email protected]:5432/ekklesia
 
+# Ekklesia's object-storage provider (ADR 0030). Absent S3_ENDPOINT means this installation has no
+# object storage, which is a complete deployment: only a module that declares a logical bucket in its
+# manifest is affected, and it is reported blocked rather than breaking anything. Production requires
+# https, exactly as the database transport requires verified TLS.
+#
+# These are host infrastructure. No module ever sees them: a module names a logical bucket and the
+# host resolves the binding.
 S3_ACCESS_KEY=ekklesia-development
 S3_SECRET_KEY=ekklesia-development-secret
-S3_BUCKET=ekklesia-development
 S3_ENDPOINT=http://127.0.0.1:8333
 S3_PORT=8333
 S3_REGION=us-east-1
 S3_FORCE_PATH_STYLE=true
+# Namespaces every physical bucket this installation owns, so several Ekklesia-compatible deployments
+# can share one S3-compatible service. Optional; defaults to "ekklesia".
+S3_BUCKET_PREFIX=ekklesia
+# Consumed only by the development Compose service's own bootstrap, never by Ekklesia's provider —
+# physical buckets are derived from a module's manifest declaration, not configured here.
+S3_BUCKET=ekklesia-development