Add sync engine: cursors, push/pull, and the advisory-lock ordering proof (M1)

The vault write path. Push is the only way items change — no per-entity POST/PUT/DELETE —
so one place enforces revisions, the change log and access control.

The concurrency hazard, now proven rather than asserted:
bigserial assigns sequence values when the INSERT runs, not at commit, so transaction A
can take sequence 5 while B takes 6 and commits first. A reader polling in between sees
only 6, advances past 5, and never learns about it. AdvisoryLockOrderingTests reproduces
that gap WITHOUT the lock first — otherwise the with-lock test proves nothing, since it
would pass just as happily if the interleaving never occurred — then shows
pg_advisory_xact_lock removes it, and that 12 concurrent writers produce no gaps.

Cursors are opaque and HMAC-tagged, and carry their vault id. 29 unit tests cover the
rejections, which are the point: an accepted-but-wrong cursor is silent data loss, strictly
worse than an error a client can resync from. Rejected: tampered tag, tampered payload,
foreign signing key, a legitimately-issued cursor from another vault, truncation, and
hostile input (never throws — cursors come from clients).

Push semantics:
- 200 even on partial failure, with per-operation status, so one stale item cannot block
  everything a client queued while offline.
- Conflict returns the server's current row for client-side three-way merge. The server
  cannot merge ciphertext, so never last-writer-wins.
- opId receipts make retries exactly-once per operation, not per batch — a client retrying
  a partially-overlapping batch after a timeout would otherwise double-apply what landed.
- A tombstone beats a late upsert, and delete clears hostname/port: leaving the address
  would keep the server able to resolve a host the user believes they deleted.
- Relay field validation mirrors the DB CHECK so a bad request is a clear Invalid rather
  than a constraint violation surfacing as a 500.

Authorization goes through IVaultAccessService, which returns the same answer for "absent"
and "forbidden" — distinguishing them is an existence oracle for other tenants' vault ids.
Team vaults are explicitly denied until M3 rather than falling through to a permissive
default. JIT provisioning keys on (issuer, subject), never email, and handles the
concurrent-first-request race via the unique index.

Renamed two domain types: Host -> SshHost, because Host collides with
Microsoft.Extensions.Hosting.Host in every file of a web project, and SyncChange ->
VaultChange to stop it colliding with the Contracts DTO of the same name. Aliasing at every
use site would have been permanent friction.

Worth noting: `ef migrations has-pending-model-changes` reported clean after those renames
even though the snapshot still said "DodoSSH.Domain.Host" — it diffs tables, not CLR type
names. The snapshot was regenerated and the emitted DDL diffed against the previous
artifacts/schema/v0.1.sql to confirm the rename produced no schema change.

Also removed ConfigureAwait(false) from test methods: xUnit1030 flags it as bypassing
parallelization limits, which is why MA0004 is suppressed in test projects.

Verified: 0 warnings on a clean rebuild, 146 tests pass (up from 122), format clean.

Endpoint-level tests are the immediate next step: they need a WireMock OIDC/JWKS stub and
real JWT minting, so the "wrong user is denied" matrix does not exist yet for these two
routes. The service-layer authorization and the concurrency property are covered.

src/DodoSSH.Infrastructure/Configurations/HostAndSyncConfigurations.cs

@@ -4,11 +4,11 @@ using Microsoft.EntityFrameworkCore.Metadata.Builders;
 
 namespace DodoSSH.Infrastructure.Configurations;
 
-/// <summary>Maps <see cref="Host"/>.</summary>
-public sealed class HostConfiguration : IEntityTypeConfiguration<Host>
+/// <summary>Maps <see cref="SshHost"/>.</summary>
+public sealed class HostConfiguration : IEntityTypeConfiguration<SshHost>
 {
     /// <inheritdoc />
-    public void Configure(EntityTypeBuilder<Host> builder)
+    public void Configure(EntityTypeBuilder<SshHost> builder)
     {
         ArgumentNullException.ThrowIfNull(builder);
 
@@ -49,11 +49,11 @@ public sealed class HostConfiguration : IEntityTypeConfiguration<Host>
     }
 }
 
-/// <summary>Maps <see cref="SyncChange"/>.</summary>
-public sealed class SyncChangeConfiguration : IEntityTypeConfiguration<SyncChange>
+/// <summary>Maps <see cref="VaultChange"/>.</summary>
+public sealed class SyncChangeConfiguration : IEntityTypeConfiguration<VaultChange>
 {
     /// <inheritdoc />
-    public void Configure(EntityTypeBuilder<SyncChange> builder)
+    public void Configure(EntityTypeBuilder<VaultChange> builder)
     {
         ArgumentNullException.ThrowIfNull(builder);