v0.174.0 — R-82 Slice B: one quiesce window, two backup tiers

MinAgent UNCHANGED — degrades gracefully against ANY older agent.

The agent gained per-target tiers in v0.97.0. The controller owns quiescing,
so the multi-tier schedule is reconciled here: every due tier is collected up
front and run inside ONE quiesce window (one stop, N sequential backups, one
resume). Two cycles on the weekly night would mean two app outages for one
night's work.

Dedup rule: local-only -> one quiesce; PBS-only -> one quiesce; BOTH due ->
ONE window with both backups inside; neither -> no quiesce.

- quiesce.TieredBackend + BackupTier + ErrTiersUnsupported (optional extension)
- agentapi: BackupTiers/BackupDueFor/StartBackupFor/BackupStatusFor;
  targetQuery("") yields an EMPTY suffix so untargeted hits the pre-R-82 route
  byte-for-byte
- Loop.resolveDueTiers = the dedup rule in one place, agent order preserved
- quiesceAndPollTiers + pollTier: app stays quiesced until the LAST tier
  snapshots (resuming earlier loses app-consistency on the DR tier). Consequence
  stated in the docs: both-due-night downtime = first tier's full backup + last
  tier's snapshot, which is why tiers run fast-first.
- Manual 'Mentes most' covers EVERY tier, due-ness ignored.
- Window-gate safety valve now uses the OLDEST due tier, so a stale DR tier
  cannot be starved by a fresher local one.

Capability detection: /backup/tiers 404 = pre-R-82 agent (the documented
route-probe mechanism). Not a featureProbes row on purpose — the loop needs the
tier LIST, not a yes/no. Degrade logged exactly once per process.

Tests +11, full suite green. Red-proofs #2 and #3 observed and restored.

REUSE.md

@@ -72,6 +72,8 @@
 | `infra.SambaContainerName` / `SambaPassdbVolume` / `SambaPassdbMount` | controller/internal/infra/samba.go | consts | single source of truth for the samba container identity | the compose renderer interpolates them; stacks/backup/monitor read them. The CONTAINER name (`felhom-samba`) is NOT the stack name (`samba`) — `EffectiveProtected` needs the container one |
 | `sambaWriteAtomic` | controller/internal/stacks/samba.go | `(path, data, mode) error` | samba smb.conf/compose writes | tmp+**fsync**+rename (the only one of these that fsyncs). Fourth atomic-write helper in the tree — see §6 |
 | `Loop.writeMarker` / `Recover` | controller/internal/quiesce/quiesce.go | `(m Marker)` / `()` | Quiesce crash-safety | Marker written BEFORE stopping stacks; Recover restarts stranded stacks at boot |
+| `quiesce.TieredBackend` + `Loop.resolveDueTiers` / `quiesceAndPollTiers` | controller/internal/quiesce/tiers.go, quiesce.go | `Tiers/DueFor/StartBackupFor/BackupStatusFor`; `resolveDueTiers(ctx) ([]dueTier,bool,error)` | THE R-82 multi-tier backup schedule — several whole-guest tiers (local daily + PBS weekly) reconciled into ONE quiesce window | **Both tiers due ⇒ ONE stop/start pair**, never two (two = two app outages for one night). Tiers run SEQUENTIALLY (vzdump holds a guest lock) and the app stays down until the LAST tier snapshots — resuming earlier loses app-consistency on the DR tier. Order is fast-first (agent advertises primary first) or downtime blows up. `ErrTiersUnsupported` (route 404) ⇒ pre-R-82 agent ⇒ degrade to the untargeted path and **STILL BACK UP** — never read it as "nothing due". |
+| `agentapi.BackupTiers` / `BackupDueFor` / `StartBackupFor` / `BackupStatusFor` | controller/internal/agentapi/backup_tiers.go | `(ctx[, target]) (…, error)` | The per-tier agent surface (agent >= v0.97.0) | `targetQuery("")` returns an EMPTY suffix so an untargeted call hits the pre-R-82 route byte-for-byte. `BackupTiers` maps a 404 to `ErrTiersUnsupported` — the documented ROUTE-PROBE capability signal, NOT a `featureProbes` row (the loop needs the tier LIST, not a yes/no). |
 
 ### Compose ops / stack lifecycle