fix(69): auto-tune review fixes — null channels, orphaned SmartCollection, oversized preview names, doc drift

- CreateAutoTunedChannelsRequest.ToCommand(): guard null Channels (was NREing on
  a request body that omits "channels", causing HTTP 500).
- CreateAutoTunedChannelsHandler.CreateOne: when CreateChannelFromLineup returns
  Left (Skipped/Failed), roll back the just-created SmartCollection via
  DeleteSmartCollection so retries don't fail on SmartCollection-name uniqueness.
  Best-effort; the delete result does not change the outcome.
- PreviewAutoTuneChannelsHandler: filter out proposals whose generated name
  exceeds the 50-char Channel.Name limit before number allocation, so numbers
  aren't wasted on proposals that can never be created.
- docs/superpowers/specs/2026-07-16-auto-tuning-design.md: fix field-name drift
  in JSON examples (proposedNumber -> number, error -> reason) to match the
  actual AutoTuneProposal/AutoTuneChannelOutcome DTOs.

Refs #69

docs/superpowers/specs/2026-07-16-auto-tuning-design.md

@@ -87,9 +87,9 @@ Response — a list of proposed channels:
 ```
 [
   { "axis": "TvShow", "value": "The Office", "name": "The Office",
-    "proposedNumber": 500, "itemCount": 201, "alreadyExists": false },
+    "number": 500, "itemCount": 201, "alreadyExists": false },
   { "axis": "MovieGenre", "value": "Action", "name": "Action Movies",
-    "proposedNumber": 501, "itemCount": 42, "alreadyExists": false },
+    "number": 501, "itemCount": 42, "alreadyExists": false },
   ...
 ]
 ```
@@ -117,7 +117,7 @@ Response — partial-success list, mirroring the existing `ResetAllPlayoutsResul
   "results": [
     { "name": "The Office", "status": "Created", "channelId": 88 },
     { "name": "Action Movies", "status": "Skipped", "reason": "number 501 already taken" },
-    { "name": "Sci-Fi", "status": "Failed", "error": "…" }
+    { "name": "Sci-Fi", "status": "Failed", "reason": "…" }
   ],
   "createdCount": 1, "skippedCount": 1, "failedCount": 1
 }