feat(api): forward exact_lang to the service context

The remote-dl client sends exact_lang in dl_params, and REST download jobs set it in the service ctx, so a service can skip the manifests of close-match languages.

docs/dev/rest-api/endpoints.md

@@ -316,7 +316,7 @@ Show the video, audio, and subtitle tracks for a title. For series, you can scop
 | `part` | int/string | no | Part index of a split episode (combined with `season` and `episode`). |
 | `profile`, `proxy`, `no_proxy`, `cdm_type` | - | no | As on `list-titles`. |
 | `credentials`, `cookies`, `cache` | - | no | As on `list-titles`. |
-| `dl_params` | object | no | The `dl` track selection that the service reads from `ctx.parent.params`: `lang`, `v_lang`, `a_lang`, `acodec` and `forced_subs`, as on [`POST /api/session/create`](#post-apisessioncreate). Some services select the manifests they fetch from these values. |
+| `dl_params` | object | no | The `dl` track selection that the service reads from `ctx.parent.params`: `lang`, `v_lang`, `a_lang`, `acodec`, `forced_subs` and `exact_lang`, as on [`POST /api/session/create`](#post-apisessioncreate). Some services select the manifests they fetch from these values. |
 
 When you give both `season` and `episode`, the server combines them into a `"{season}x{episode}"` selector. `part` narrows that to `"{season}x{episode}.{part}"`, one part of a [split episode](../creating-a-service.md#split-episodes). Without `part` you get every part of the episode. The server reads `part` only when you give both `season` and `episode`, and `wanted` takes precedence over all three. To scope a part by range, put it in `wanted` instead (`"S01E01.2"`).
 
@@ -958,7 +958,7 @@ the newlines must go out at once.
 
 Make a remote session for a service and title. Authentication runs asynchronously in the background. This call returns immediately with `status: "authenticating"`, and you then poll the prompt endpoint. The body accepts `service` and `title_id` (both required). It also accepts a broad set of optional keys, because the body allows `additionalProperties`. These are `credentials` (`{username, password, extra?}`), `cookies` (base64 of zlib-compressed Netscape cookie file), `proxy`, `no_proxy`, `profile`, `cache` (map of forwarded cache files, keyed by the file path relative to the service cache directory with `/` separators and no `.json` suffix, so a nested key such as `session_web/<sha1>` lands in a subdirectory), `client_region`, `proxy_region`, `cdm_type`, `range_`, `vcodec`, `quality`, `best_available`, `dl_params`, `client`, and any service CLI options. `proxy_region` is the country code the client resolved `proxy` from; the server matches it against its own accounts. `client` is a freeform object the dashboard shows as sent (the CLI puts `version`, `code_hash`, `platform` and a redacted `argv` in it); the server ignores it above 4096 bytes of JSON.
 
-`dl_params` is an object with the `dl` track selection: `lang`, `v_lang` and `a_lang` (arrays of language strings), `acodec` (an array of audio codec names or `dl -a` aliases such as `ddp`) and `forced_subs` (boolean). The service reads these values from `ctx.parent.params`. The server uses the `dl` default for each absent or malformed value, and drops an unknown codec name.
+`dl_params` is an object with the `dl` track selection: `lang`, `v_lang` and `a_lang` (arrays of language strings), `acodec` (an array of audio codec names or `dl -a` aliases such as `ddp`), and `forced_subs` and `exact_lang` (booleans). The service reads these values from `ctx.parent.params`. The server uses the `dl` default for each absent or malformed value, and drops an unknown codec name.
 
 Service CLI options also travel in a nested `service_params` object, which wins over a flat key with the same name. `profile` at the top level always means the credentials profile, never a service's own `--profile` option.
 

docs/dev/rest-api/remote-sessions.md

@@ -432,11 +432,11 @@ object the dashboard shows as sent), and the track-selection hints `range_`, `vc
 
 `dl_params` holds the `dl` track selection: `lang`, `v_lang` and `a_lang`
 (arrays of language strings), `acodec` (an array of audio codec names, such as
-`EC3`), and `forced_subs` (boolean). The server gives these values to the
+`EC3`), and `forced_subs` and `exact_lang` (booleans). The server gives these values to the
 service in `ctx.parent.params`, never as service options, so a service option
 with the same name, such as an own `--lang`, does not get them. The server uses
 the `dl` default for each absent or malformed value: `lang` is `["orig"]`,
-`v_lang`, `a_lang` and `acodec` are `[]`, and `forced_subs` is `false`. The
+`v_lang`, `a_lang` and `acodec` are `[]`, and `forced_subs` and `exact_lang` are `false`. The
 server drops an unknown codec name. An empty array is a valid value and stays
 empty.