domain glossary, context map, ADRs (library-as-source, playout, event queue)

crates/docs/adr/0002-ktv-owns-the-stream.md

@@ -0,0 +1,31 @@
+# ADR-0002: K-TV owns the stream via a Playout Service
+
+## Status
+
+Accepted
+
+## Context
+
+The original design proxied streaming to each provider — Jellyfin served its own HLS, local files were either served directly or transcoded individually. This made it impossible to:
+
+- Inject SCTE-35 markers for mid-roll breaks
+- Insert timed metadata for overlays
+- Stitch interstitial content between program items into a continuous stream
+- Guarantee consistent stream format across providers
+
+## Decision
+
+K-TV owns the stream end-to-end via a Playout Service. Providers expose a source URI (network URL or local file path) instead of a viewer-facing playback URL. The Playout Service reads from the source URI using FFmpeg on-demand (no full file download), and produces the HLS output with all audio/subtitle tracks, segmentation, SCTE-35, and timed metadata.
+
+## Alternatives considered
+
+- **Continue proxying to provider streams** — rejected because mid-roll breaks, interstitials, and overlays require playlist-level control that provider-owned streams don't offer.
+- **Download files locally then transcode** — rejected because it duplicates storage and defeats the purpose of having providers manage content.
+
+## Consequences
+
+- Provider port changes from "give me a playback URL" to "give me a source URI."
+- Every stream goes through FFmpeg — CPU cost scales with concurrent viewers. Caching segments mitigates repeat access.
+- Subtitles and audio tracks come from the source container — no separate subtitle API needed.
+- All viewers get a uniform HLS experience regardless of provider.
+- Disk space management becomes critical — HLS segments must be cleaned up.