Implement edge-host-self-enrollment

Host Agent:
- One-time local operator account bootstrap (PBKDF2-HMAC-SHA256, atomic
  0600-permission write) gating the daemon's first unattended start via a
  new `setup` CLI subcommand.
- Default control-plane URL now https://amcp.home.jerryyan.top (env var
  override unchanged).
- Enrollment no longer requires a pre-issued token; falls back to
  zero-token self-service enrollment when none is configured.

Cloud control plane:
- CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED (default false) opt-in flag.
- SelfServiceEnrollmentAuthProvider + ChainedEnrollmentAuthProvider:
  configured tokens still take priority; self-service only applies when
  no token matches, preserving edge-host-enrollment's token-bound path.
- Fixed a latent bug in sql_repository.py::enroll_host: the token-conflict
  lookup used `== enrollment_token_digest`, which SQLAlchemy compiles to
  `IS NULL` when the value is None, so every self-service enrollment after
  the first would have falsely collided with an existing NULL-digest host.
  Skipped that lookup entirely when the digest is None.

Docs/deploy: .env.example, compose.yaml, compose.deploy.yaml,
CLOUD_DEPLOYMENT.md, MACOS_IPHONE_SETUP.md updated for the new flag,
URL default, and required `device-host-agent setup` step.

Verification: 494 non-integration tests pass; openspec validate --strict
passes. PostgreSQL-backed contract tests and full manual end-to-end
verification were not run (no Postgres/Docker or reachable cloud-api in
this environment); noted as unchecked in tasks.md 7.2/7.4.

apps/cloud-api/cloud_api/app.py

@@ -18,8 +18,10 @@ from starlette.types import Scope
 
 from cloud.auth import (
     ChainedAuthProvider,
+    ChainedEnrollmentAuthProvider,
     ConfiguredEnrollmentTokenProvider,
     RepositoryHostAuthProvider,
+    SelfServiceEnrollmentAuthProvider,
     UserSessionAuthProvider,
     create_auth_provider,
 )
@@ -134,8 +136,15 @@ def create_app(
             RepositoryHostAuthProvider(repository),  # type: ignore[arg-type]
         )
     )
-    enrollment_auth_provider = ConfiguredEnrollmentTokenProvider(
-        control_config.enrollment_credentials
+    enrollment_auth_provider = ChainedEnrollmentAuthProvider(
+        (
+            ConfiguredEnrollmentTokenProvider(control_config.enrollment_credentials),
+            *(
+                (SelfServiceEnrollmentAuthProvider(),)
+                if control_config.self_service_enrollment_enabled
+                else ()
+            ),
+        )
     )
     domain_config = CloudConfig(
         lease_duration_seconds=control_config.lease_duration_seconds,