hive-tls: issue the gateway leaf from bao with a per-hive identity

Every hive gets its own store identity (CN
<deploy.bao.hiveLeafCommonNamePrefix>-<hive>) and its own role on the
existing pki mount (<deploy.bao.hiveLeafPkiRolePrefix>-<hive>), whose
allowed_domains is the hive's domain and *.<domain>. The policy grants
update on that one issue path. swarm-bao-hive-leaf-policy writes role,
policy and cert-auth role for every hive in swarm.hives; the store's host
mints its own hive's identity in glue-bao-tls.nix and a new glue file
points deploy.hive-controller.tls.baoHiveIdentity{Cert,Key}File at it.
Hives on other hosts place it by hand. swarm.nix reserves the composed CN.

hive-tls-ca no longer generates a hive CA or signs a leaf: caGenScript,
signLeafScript/signHiveLeaf and the adoption block are gone, ca-key.pem
is deleted where it exists, and the unit only assembles trust-bundle.pem.
The retired ca.pem/ca-previous.pem stay in the bundle so leaves they
signed keep verifying.

hive-tls-resign becomes the bao issuance: at boot, on deploy and from a
daily timer it re-issues when the leaf is missing, past half the role's
lifetime, missing a name, or not chained to the store's pki root. Nothing
orders the gateway after it, so a valid leaf on disk never waits on the
store; a failed renewal leaves the leaf in place. A missing identity
fails the unit naming both options, as does the root fetch on a hive
fronting no swarm name, which now presents this identity instead of the
reader cert.

caValidityDays and leafValidityDays are removed (both spellings), since
nothing reads them.

Refs #4588

docs/swarm/README.md

@@ -33,7 +33,7 @@ grafana, the controller, authelia, …) →
   lives → [`secrets.md`](secrets.md) · the per-secret minter/reader/renewal
   contract → [`credentials.md`](credentials.md) · how the store comes up,
   who writes its grants and how it unseals → [`bao.md`](bao.md)
-- **swarm CA** — the root every hive's internal TLS chains to, and what to
+- **swarm CA** — the store root every hive's gateway leaf chains to, and what to
   hand a peer (`trust-bundle.pem`, never `ca.pem`). → [`ca.md`](ca.md)
 - **snapshot store** — the swarm's one `btrfs receive` endpoint,
   `swarm.snapshotStore.{address,port}`. →