docs: add architecture decision summary for interview (#250)

ARCHITECTURE_DECISIONS.md

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+# Architecture Decisions
+
+Short summary of core design choices and their rationale.
+For full documentation, run `just book` to view the rendered doc site.
+
+## System Shape
+
+**Microservice boundaries, single-process delivery.**
+The system is split into four services — Gateway, MetaService, Ingestor, Streamer — each with its own crate and API surface. In the submitted build they all run inside one process (`stream-app`) backed by SQLite, an in-memory queue, and an in-memory object store. This keeps deployment trivial while preserving the ability to split services later without API changes.
+
+## Control Plane vs Data Plane
+
+**Gateway proxies metadata; streaming bypasses it.**
+Gateway handles auth, rate limiting, and proxies requests to MetaService and Ingestor. Streamer is accessed directly by clients — video data should not pass through a reverse proxy. Access control for the data plane uses short-lived HMAC-signed connection tokens issued by MetaService, scoped by action (upload / stream) with a 5-minute TTL.
+
+## Upload & Processing
+
+**Async pipeline with crash recovery.**
+Uploads land in object storage immediately; transmux to HLS happens asynchronously via a task queue. Processing state is persisted in the database so incomplete jobs resume after a crash instead of starting over. Transient errors (I/O timeout) retry with exponential backoff; permanent errors (corrupt file) fail fast.
+
+## Playback
+
+**Streamer is a pure cache layer.**
+Streamer fetches HLS segments from object storage on first request and caches them locally. It does no transcoding — that responsibility belongs to Ingestor. This separation keeps playback latency low and lets Streamer scale independently on bandwidth.
+
+## Share Links & Access Control
+
+**Two-layer authorization: share link + connection token.**
+Share links control *who can enter* (owner can revoke instantly). Connection tokens control *how long they stay* (5-min TTL limits replay window). If a link leaks, revocation cuts off access within one token lifetime.
+
+## Client
+
+**Rust/WASM core + Svelte UI shell.**
+Core logic (hashing, token management, validation) lives in Rust compiled to WASM, shared with the backend to prevent divergence. The Svelte shell handles routing and UI. Client-side format validation is a UX fast-fail; the real security boundary is server-side codec probing in Ingestor.
+
+## Key Trade-offs
+
+| Decision | Trade-off | Why |
+|----------|-----------|-----|
+| Single-process mode | No horizontal scaling | Simplifies deployment for assignment; service boundaries are ready for split |
+| In-memory object store | Data lost on restart | Avoids external infra dependency; swappable to S3 via config |
+| No auth system | No real user identity | Assignment spec says auth not required; Gateway middleware slot is ready for JWT |
+| WASM client | Larger initial bundle | Reuses Rust logic; avoids client/server divergence on hashing and validation |
+| HLS (not DASH) | Apple-centric format | Broadest native browser support; simpler transmux pipeline with ffmpeg |