feat: add retry handling for transient sync failures

CHANGELOG.md

@@ -28,6 +28,21 @@ This project follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   note body content, passwords, tokens, or server URLs — error strings are
   scrubbed and the plan/report only ever hold metadata.
 
+- **Retry & backoff for transient failures.** Every WebDAV network
+  operation (GET, PROPFIND, PUT, MKCOL) is now retried on transient
+  conditions — timeouts, connection drops, and HTTP 429/502/503/504 —
+  with small conservative defaults (3 attempts, 0.5s/1s/2s backoff).
+  Auth failures (401/403) and 404 are never retried. Both the attempt
+  budget and per-call timeout are configurable on `SyncConfig`
+  (`max_attempts`, `backoff_seconds`, `timeout_seconds`).
+
+- **Retryable sync reports.** When a sync fails for a transient reason the
+  report (and `POST /sync/trigger`) carries `retryable: true`, a short
+  `transient_reason`, and a suggested `next_retry_after_seconds`. The
+  diagnostic log additionally records per-operation attempt counts so a
+  flaky NAS/Cloudflare/mobile link is easy to spot — still with no body
+  content, credentials, or server URLs.
+
 ### Improved
 
 - **Conflict safety.** When a note changed both locally and remotely, the

nexanote/api/routes.py

@@ -176,6 +176,12 @@ class SyncReportSchema(BaseModel):
     conflicts: list[dict] = Field(default_factory=list)
     warnings: list[str] = Field(default_factory=list)
     plan: dict = Field(default_factory=dict)
+    # Network reliability — additive too. ``retryable`` flags a transient
+    # failure (timeout/connection/429/502/503/504) so clients can back off
+    # and retry instead of surfacing it as a hard error.
+    retryable: bool = False
+    next_retry_after_seconds: Optional[float] = None
+    transient_reason: Optional[str] = None
 
 
 class ExportRequestSchema(BaseModel):
@@ -603,6 +609,9 @@ def trigger_sync(dry_run: bool = Query(False)):
             conflicts=[c.to_dict() for c in plan.conflicts] if plan else [],
             warnings=list(plan.warnings) if plan else [],
             plan=plan.to_dict() if plan else {},
+            retryable=report.retryable,
+            next_retry_after_seconds=report.next_retry_after_seconds,
+            transient_reason=report.transient_reason,
         )
         # A dry-run is a preview — it must not clobber the last *real* status.
         if not dry_run: