docs: add spec for pipeline tweaks (parallel extraction, shuffle, clear history)

docs/superpowers/specs/2026-03-25-pipeline-tweaks-design.md

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+# Design: Pipeline Tweaks — Parallel Extraction, Shuffle, Clear History
+
+**Date**: 2026-03-25
+**Scope**: 4 focused improvements to the synthesis pipeline and article history UI
+
+---
+
+## 1. Remove max source limit + extract 15 links
+
+Currently `max_sources = rotated_sources.len().min(10)` limits to 10 sources, and `max_links = 10` limits to 10 links per source.
+
+Change to:
+- Process ALL user sources (no `.min(10)` cap)
+- Extract 15 links per source (`max_links = 15`)
+
+## 2. Parallel source extraction (concurrency 5)
+
+Currently source pages are scraped sequentially in a `for` loop. Change to use `JoinSet` with max 5 concurrent extractions, same pattern as article scraping.
+
+Both `extract_article_links` and `extract_article_links_with_llm` are async and return `Result<Vec<String>>`. The parallel loop spawns tasks and collects results.
+
+## 3. Shuffle candidates after dedup/history filter
+
+After deduplication and history filtering, before url→source tracking, shuffle `candidate_urls` using `rand::thread_rng()`. This ensures articles from different sources are interleaved rather than processed source-by-source.
+
+The `rand` crate is already a dependency.
+
+## 4. Clear history button
+
+New API endpoint: `DELETE /api/v1/article-history` — deletes ALL article_history entries for the authenticated user.
+
+New DB function: `delete_all_for_user(pool, user_id)`.
+
+Frontend: "Effacer l'historique" button on the ArticleHistory page with a confirmation dialog.
+
+## Files to Modify
+
+- **Modify:** `backend/src/services/synthesis.rs` — remove source cap, change to 15 links, parallel extraction with JoinSet, add shuffle
+- **Modify:** `backend/src/db/article_history.rs` — add `delete_all_for_user`
+- **Create:** handler for DELETE endpoint (add to existing `article_history.rs` handler)
+- **Modify:** `backend/src/router.rs` — add DELETE route
+- **Modify:** `frontend/src/api/articleHistory.ts` — add `clearAll` method
+- **Modify:** `frontend/src/pages/ArticleHistory.tsx` — add clear button with confirmation
+- **Modify:** `frontend/src/i18n/fr.ts` — add labels