(feature) Add host backup preflight gates

Introduce a host preflight layer that separates dry-run blockers from real backup blockers.
Show the effective per-host backup configuration in Django before queueing a run.

Block real backup queueing when failed host checks remain, while still allowing dry-runs
when only local storage preparation is missing.

src/pobsync_backend/preflight.py

@@ -0,0 +1,100 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+from pobsync.config.merge import build_effective_config
+
+from .config_repository import global_config_object_data, host_config_object_data
+from .host_ops import collect_host_checks
+from .models import GlobalConfig, HostConfig
+from .self_check import SelfCheck
+
+
+DRY_RUN_BLOCKING_CHECKS = {
+    "Host global config",
+    "Host address",
+    "Host SSH key file",
+    "Host effective source root",
+    "Host effective SSH user",
+    "Host effective SSH port",
+    "Host effective SSH credential",
+    "Host effective rsync recursion",
+}
+
+
+@dataclass(frozen=True)
+class BackupGate:
+    state: str
+    message: str
+    checks: list[SelfCheck]
+    real_blockers: list[SelfCheck]
+    dry_run_blockers: list[SelfCheck]
+    warnings: list[SelfCheck]
+
+    @property
+    def can_queue_real(self) -> bool:
+        return not self.real_blockers
+
+    @property
+    def can_queue_dry_run(self) -> bool:
+        return not self.dry_run_blockers
+
+
+def collect_backup_gate(host: HostConfig, global_config: GlobalConfig | None = None) -> BackupGate:
+    checks = collect_host_checks(host, global_config)
+    real_blockers = [check for check in checks if check.status == "failed"]
+    dry_run_blockers = [check for check in real_blockers if check.name in DRY_RUN_BLOCKING_CHECKS]
+    warnings = [check for check in checks if check.status == "warning"]
+
+    if real_blockers:
+        state = "blocked"
+        message = "Real backups are blocked until failed host checks are resolved."
+    elif warnings:
+        state = "warning"
+        message = "Backups can run, but review the warnings first."
+    else:
+        state = "ready"
+        message = "This host is ready for backup runs."
+
+    return BackupGate(
+        state=state,
+        message=message,
+        checks=checks,
+        real_blockers=real_blockers,
+        dry_run_blockers=dry_run_blockers,
+        warnings=warnings,
+    )
+
+
+def effective_host_config_preview(host: HostConfig, global_config: GlobalConfig) -> dict[str, Any]:
+    config = build_effective_config(global_config_object_data(global_config), host_config_object_data(host))
+    credential = host.ssh_credential or global_config.default_ssh_credential
+    ssh = config.get("ssh", {}) or {}
+    rsync = config.get("rsync", {}) or {}
+    retention = config.get("retention", {}) or {}
+
+    return {
+        "source_root": config.get("source_root", ""),
+        "destination_subdir": (config.get("defaults", {}) or {}).get("destination_subdir", ""),
+        "includes": list(config.get("includes") or []),
+        "excludes": list(config.get("excludes_effective") or []),
+        "ssh": {
+            "user": ssh.get("user", ""),
+            "port": ssh.get("port", ""),
+            "options": list(ssh.get("options") or []),
+            "credential": str(credential) if credential else "",
+        },
+        "rsync": {
+            "binary": rsync.get("binary", ""),
+            "args": list(rsync.get("args_effective") or []),
+            "timeout_seconds": rsync.get("timeout_seconds", 0),
+            "bwlimit_kbps": rsync.get("bwlimit_kbps", 0),
+        },
+        "retention": {
+            "daily": retention.get("daily", 0),
+            "weekly": retention.get("weekly", 0),
+            "monthly": retention.get("monthly", 0),
+            "yearly": retention.get("yearly", 0),
+        },
+    }