docs(arr): add DESIGN.md §15 subtitles contract

Records the subtitle decisions settled during Subtitles milestone
planning: global wanted set (pt + en), embedded-track extraction,
providers, ranking, translation backends, sidecar naming, alass sync,
budgets, and stated non-goals. Points §13 item 9 at §15 and adds
arr-subs to the crate lists.

Closes-Issue: #183

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

DESIGN.md

@@ -666,6 +666,7 @@ arr-meta      TMDB client
 arr-indexer   Torznab via Prowlarr
 arr-dl        Transmission RPC
 arr-probe     ffprobe wrapper
+arr-subs      subtitle providers, translation, sync
 arr-db        sqlx + migrations
 arr-api       axum + OpenAPI
 arr-compat    Radarr/Sonarr v3 shim for Jellyseerr
@@ -748,7 +749,7 @@ previous one.
    grabbing, derived status.
 7. **Owners and notifications** — tags, per-person ntfy topics, filtered views.
 8. **Jellyseerr compat** — `arr-compat`.
-9. **Subtitles** — replaces Bazarr. Out of scope for this document.
+9. **Subtitles** — replaces Bazarr. See §15.
 
 Movies before TV because TV adds season packs, air-date calendars and
 per-episode state on top of an otherwise identical pipeline. Doing it second
@@ -768,3 +769,88 @@ means that pipeline is already proven.
   RSS lane obeys the same guard — it skips packs for any season with episodes
   on disk. Whether a re-grab is ever wanted remains unresolved and deliberately
   deferred.
+
+## 15. Subtitles
+
+Replaces Bazarr. Phase 9 in §13; the `arr-subs` crate in §11.
+
+**Wanted set.** Global, not per root. Two languages are separately wanted for
+every media file: Portuguese — pt-PT preferred, pt-BR accepted — and English.
+A file is satisfied for a language when a subtitle in it exists, embedded or as
+a sidecar. This is deliberately unlike §5.2's audio rules, which attach to a
+root: subtitles carry no blacklist there and none here. pt-BR subtitles are
+always fine.
+
+**Embedded tracks.** An embedded subtitle track satisfies its language.
+Text-format tracks (`subrip`, `ass`, `mov_text`) are additionally extracted to
+a sidecar SRT, because an extracted track is a legal translation source.
+Image-format tracks (PGS on BluRay, VobSub on DVD) carry bitmaps, not text:
+they satisfy viewing but can never feed a translator, and arr does not OCR
+them. `arr-probe` already reports subtitle tracks with resolved languages; the
+format is the new fact it must carry.
+
+**Providers.** OpenSubtitles.com and Podnapisi, behind one trait.
+
+**Ranking.** A `moviehash` match wins outright. Then an exact release-name
+match, then same release group or same source, then uploader rating and
+download count as tiebreakers. Every rejected candidate names the rule that
+killed it, so the manual view described in §9.3 works unchanged for subtitles.
+
+**Forced and SDH.** A forced track covers only foreign-language lines and
+on-screen signs; it never satisfies a want and arr never goes looking for one.
+An SDH track is complete and satisfies, ranked below a plain subtitle.
+
+**Translation.** When no provider has a wanted language, arr translates
+immediately — there is no waiting window. The source is an existing subtitle:
+a downloaded one, or one extracted from a text-format embedded track. Being
+able to translate from an embedded track is a deliberate improvement on
+Bazarr, which cannot.
+
+**Translation backends.** Pluggable, each behind its own cargo feature: an
+OpenAI-compatible HTTP endpoint, DeepL, Google Translate, and a generic remote
+command driven by a configured template (`ssh box claude -p` is one instance
+of that template, not a backend of its own). The engine in use is a database
+setting, so switching does not need a rebuild when the feature is compiled in.
+Subtitles are sent in batches of cues; a reply whose cue count or numbering
+does not match the batch is rejected. Timing data never leaves arr.
+
+**No upgrade loop.** Once a language is satisfied — by a machine translation
+too — arr stops working on it. A real subtitle appearing later does not
+replace anything. Replacement is a manual action from the UI. This is §5.4's
+rule applied to subtitles.
+
+**On disk.** Sidecars live next to the video inside the §7.4 title folder,
+named `<video basename>.<lang>.srt`, e.g.
+`… - [2160p][WEB-DL][HDR10].pt-PT.srt`. A machine translation carries an extra
+`.mt` segment: `… [HDR10].pt-PT.mt.srt`. That keeps §7.4's audit-by-`ls`
+property — with the app stopped, the filename says which subtitles are
+machine-made. Folder-level delete stays atomic because sidecars are inside the
+folder.
+
+**Sync.** `alass` runs on every fetched and every translated subtitle. It is a
+single small binary invoked like `ffprobe`, so it costs nothing at rest. It
+reports no confidence value, so its output is accepted unless it is
+implausible — a shift beyond 60 seconds, or cues lost — in which case the
+unsynced original is kept and the file is flagged.
+
+**Configuration.** Provider credentials and translator API keys are bootstrap
+config or environment, per §10. Wanted languages, chosen engine, per-provider
+enable and the daily budgets are database rows edited from `/settings` without
+a restart.
+
+**Budgets.** A token bucket per provider and per translator, with a configured
+daily allowance. The reconcile loop spends it newest-import-first, so enabling
+this on an existing library drains the backlog over days instead of hitting
+every rate limit at once. Being at the cap is a visible queue state, not an
+error.
+
+**Notifications.** No new event classes. §9.5 stands: subtitle fetches never
+notify, and a provider or translator being unreachable folds into the existing
+"Broken" message to the operator alone.
+
+**Non-goals**, stated here so they do not creep back: OCR of image-based
+tracks, transcribing audio when no subtitle exists anywhere, adopting subtitle
+files already on disk that arr did not write (§2 already says the service
+knows only what it put there — which does mean arr may fetch a second copy
+alongside one Bazarr left), and any background loop that upgrades a subtitle
+in place.