fix: collapse duplicate leaderboard entries for the same docs URL - #39

Summary

developer.salesforce.com/docs ended up with two rows: the curated salesforce entry and a URL-derived developer-salesforce-com-docs entry, each carrying a different grade for the same site. Any curated entry whose docsUrl has a path hits this — the slug resolution in POST /api/score only reuses a name-derived slug when the URL has no path, so a form submission of the same URL always mints a second slug:

const rawSlug = slugParam || (effectiveName && !urlPath && !isFernHost ? nameToSlug(effectiveName) : urlToSlug(url));
//                                              ^^^^^^^^ "/docs" → falls through to urlToSlug

Root-cause fix: look the URL up in the table first and reuse whatever slug it is already stored under, so a re-submit updates the existing entry instead of creating a sibling.

const storedSlugForUrl = slugParam ? null : await getScoreSlugByDocsUrl(url);
const rawSlug = slugParam || storedSlugForUrl || (effectiveName && !urlPath && !isFernHost ? ... );

getScoreSlugByDocsUrl matches on docs_url normalized for trailing slashes and case, and orders hidden ASC, scored_at DESC so a visible curated slug wins over a hidden auto-generated duplicate.

The already-existing duplicate row can't be fixed by that path, so it also gets a SLUG_ALIASES entry (developer-salesforce-com-docs → salesforce), which makes the shared URL redirect to the curated entry and keeps the slug from ever being scored again.

Link to Devin session: https://app.devin.ai/sessions/b0437a2b21464d4c8bd6849e8507ddf1