[AZ-137] [AZ-138] Decompose test tasks and scaffold E2E test infrastructure

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_docs/02_tasks/AZ-142_test_async_sse.md

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+# Async Detection & SSE Streaming Tests
+
+**Task**: AZ-142_test_async_sse
+**Name**: Async Detection & SSE Streaming Tests
+**Description**: Implement E2E tests verifying async media detection initiation, SSE event streaming, and duplicate media_id rejection
+**Complexity**: 3 points
+**Dependencies**: AZ-138_test_infrastructure
+**Component**: Integration Tests
+**Jira**: AZ-142
+**Epic**: AZ-137
+
+## Problem
+
+Async media detection via POST /detect/{media_id} must return immediately with "started" status while processing continues in background. SSE streaming must deliver real-time detection events to connected clients. Duplicate media_id submissions must be rejected with 409.
+
+## Outcome
+
+- Async detection returns immediately without waiting for processing
+- SSE connection receives detection events during processing
+- Final SSE event signals completion with mediaStatus "AIProcessed"
+- Duplicate media_id correctly rejected with 409 Conflict
+
+## Scope
+
+### Included
+- FT-P-08: Async media detection returns "started" immediately
+- FT-P-09: SSE streaming delivers detection events during async processing
+- FT-N-04: Duplicate media_id returns 409
+
+### Excluded
+- Video frame sampling details (covered in video tests)
+- SSE queue overflow behavior (covered in resource limit tests)
+- Annotations service interaction (covered in resilience tests)
+
+## Acceptance Criteria
+
+**AC-1: Immediate async response**
+Given an initialized engine
+When POST /detect/{media_id} is called with config and auth headers
+Then response arrives within 1s with {"status": "started"}
+
+**AC-2: SSE event delivery**
+Given an SSE client connected to GET /detect/stream
+When async detection is triggered via POST /detect/{media_id}
+Then SSE events are received with detection data and mediaStatus "AIProcessing"
+And a final event with mediaStatus "AIProcessed" and percent 100 arrives
+
+**AC-3: Duplicate media_id rejection**
+Given an async detection is already in progress for a media_id
+When POST /detect/{media_id} is called again with the same ID
+Then response is 409 Conflict
+
+## Non-Functional Requirements
+
+**Performance**
+- Async initiation response within 1s
+- SSE events delivered within 120s total processing time
+
+## Integration Tests
+
+| AC Ref | Initial Data/Conditions | What to Test | Expected Behavior | NFR References |
+|--------|------------------------|-------------|-------------------|----------------|
+| AC-1 | Engine warm, test-video, JWT token | POST /detect/{media_id} | Response < 1s, status "started" | Max 2s |
+| AC-2 | Engine warm, SSE connected, test-video | Listen SSE during async detection | Events received, final AIProcessed | Max 120s |
+| AC-3 | Active detection in progress | Second POST with same media_id | 409 Conflict | Max 5s |
+
+## Constraints
+
+- SSE client must connect before triggering async detection
+- JWT token required for async detection endpoint
+- Test video must be accessible via configured paths
+
+## Risks & Mitigation
+
+**Risk 1: SSE connection timing**
+- *Risk*: SSE connection may not be established before detection starts
+- *Mitigation*: Add small delay between SSE connect and detection trigger; verify connection established