feat(dl): add --skip-missing to skip titles that miss a required language or track

A season run skips such a title and continues; one title, or a run that skips every title, still exits 1 (now also for -ado/-fso). REST jobs report skipped_titles.

docs/reference/configuration/download.md

@@ -53,6 +53,7 @@ Common config keys. This is a useful subset, and every `dl` flag works:
 | `speed_limit` | str | *(unlimited)* | Total download speed cap across all downloads combined, e.g. `500k`, `5M`, `1.5G` or plain bytes/sec. Values are bytes, not bits (`5M` = 5.0 MB/s). |
 | `slow` | str | *(unset)* | Inter-title delay, e.g. `"20-40"`. |
 | `best_available` | bool | `false` | Warn instead of failing when a requested resolution, range, video language, or audio language is missing, or when one DRM system does not license under `--all-drm`. The CLI flag is `--best-available`, also spelled `--warn-only`. |
+| `skip_missing` | bool | `false` | Skip a title that misses a required language (`require_audio`, `require_video`, `require_subs`) or a track that a filter asks for, and continue with the next title. A run with one title, or a run that skips every title, still fails. The CLI flag is `--skip-missing`. For REST API jobs, set it in the [`serve`](services.md#serve) block. |
 | `generate_dv` | bool | `false` | With `-r HYBRID`, make Dolby Vision from the HDR10+ metadata or the decoded video when a title has no DV track. With `-r HDR10` or `-r HDR10P`, make it for that base and ignore a DV track. See [Dolby Vision without a DV track](../../guide/downloading.md#dolby-vision-without-a-dv-track). |
 | `proxy` | str | *(unset)* | Default proxy URI or 2-letter country. |
 | `no_folder` | bool | `false` | Do not create a per-title folder. |

docs/reference/configuration/post-scripts.md

@@ -213,6 +213,10 @@ subs = [p for p in sys.argv[i].removeprefix("--subs=").split("\n") if p]
     nothing about whether the season is complete on disk. Downloading a single episode fires
     the post-script for that season as soon as the episode lands.
 
+    A title that track selection skips (`--skip-missing`, `-ado`, `-fso`) counts as done. The
+    `season` post-script still fires for the episodes that landed, so the folder can be missing
+    the skipped episodes. A run that skips every title fails, and then no `season` post-script fires.
+
     A title that fails stops the whole run. unshackle dispatches the `failure` post-script for
     that title and then returns. It downloads no further title, and no `season` or `run`
     post-script fires after that point. This applies to every title type. If one episode of S01