Update architecture baseline for sync, sharing, and gateways

docs/adr/0009-external-protocol-gateway-boundary.md

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+# ADR-0009: External protocol gateway boundary
+
+Status: Accepted
+
+## Context
+
+Existing backup and automation tools may benefit from WebDAV or rclone compatibility. Managed Drive storage is not a user-visible filesystem: PostgreSQL owns hierarchy and revisions while the array contains opaque immutable blobs. Direct protocol access to object paths would bypass authorization, commit intents, change events, trash, replacement recovery, and audit.
+
+## Decision
+
+- REST and application services remain canonical.
+- A writable WebDAV gateway MAY be implemented post-v1 as a protocol translator. Every write calls normal application services, uses revision preconditions, creates normal audit/change records, and sends deletion to trash unless a credential explicitly permits permanent deletion.
+- WebDAV uses independently revocable, action- and subtree-scoped credentials. Durable WebDAV lock tables are added with that module, not pre-created in the core schema.
+- A native rclone backend MAY later call the REST API directly for richer hashes, modification times, resumable uploads, server-side move/copy, trash, and error semantics. Generic rclone-over-WebDAV and a native backend may coexist.
+- Native rsync service, rsync staging ingestion, and direct SMB/SFTP access to managed object storage are not planned. Rsync may operate on a local folder maintained by a future desktop client, never on Drive's server object directory.
+- No gateway may import storage adapters or mutate core repository tables directly (I16).
+
+Integration configuration and lock tables are introduced only when their module is implemented. Phase 1 adds only the shared foundations already required by REST clients: client identity, persistent idempotency, mutation attribution, and scoped credentials.
+
+## Consequences
+
+- Future compatibility does not force writable WebDAV or integration-specific tables into stable v1.
+- Existing tools can gain interoperability later without weakening the immutable object model.
+- A protocol gateway must provide streaming, conditional requests, bounded listings, authentication, audit, and revocation appropriate to that protocol.
+