Adds POST /v1/dedupe/stream - a streaming variant of /v1/dedupe that sends Server-Sent Events as each pipeline stage completes.

Motivation

Large dedup requests can take several seconds (especially with embedding generation). Streaming provides progress visibility and faster time-to-first-byte.

SSE event protocol

event: progress
data: {"stage":"embedding","progress":0}

event: progress
data: {"stage":"embedding","progress":1}

event: progress
data: {"stage":"clustering","progress":1,"stats":{"clusters_formed":5,"input_count":12}}

event: progress
data: {"stage":"selection","progress":1,"stats":{"selected":5}}

event: complete
data: {"chunks":[...],"stats":{"input_count":12,"output_count":5,...}}

Three event types:

• progress - stage name, completion ratio (0-1), optional per-stage stats
• complete - final chunks + stats (same schema as /v1/dedupe response)
• error - stage + error message

Usage

curl -N -X POST http://localhost:8080/v1/dedupe/stream \
  -H "Content-Type: application/json" \
  -d '{"chunks": [{"text": "..."}], "threshold": 0.15}'

const es = new EventSource('/v1/dedupe/stream', { method: 'POST', body: ... });
es.addEventListener('progress', (e) => console.log(JSON.parse(e.data)));
es.addEventListener('complete', (e) => console.log(JSON.parse(e.data)));

Changes

• pkg/sse/sse.go - SSE Writer with typed event methods (progress, complete, error), stage timer
• pkg/sse/sse_test.go - 9 unit tests
• cmd/api.go - New /v1/dedupe/stream route, streaming handler with per-stage events, tracing + metrics integration

Closes #6