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Replace the {repoID} and unscoped API routes with GitHub-style
/api/v1/repos/{owner}/{repo}/... routes, resolved case-insensitively
through the registry's owner/repo index. Branch-diff moves from a
?branch= query to a /branch-diff/{name...} path segment: branch names
contain slashes and Go's mux requires a trailing wildcard to be the
final segment, so .../branches/{name}/diff is not expressible.

Rewrite the OpenAPI spec to the scoped shape (it had drifted to the old
unscoped paths) and document the submit, sync, and events routes. Drops
the legacy default-fallback resolution. The web client moves to these
routes in the next change.

Stack

💡 Notice: This PR is part of a stack merge into branch stack-merge-stack-1782266218.

• PR refactor(web): remove sample alice/bob stacks from repo provider #1313
  • PR feat(server): drive deploy posture from STACKIT_ENV, not $PORT #1314
    • PR refactor(api): extract Syncer.SyncRepo for per-repo sync triggers #1315
      • PR feat(api): add GitHub webhook signature + push parsing helper #1316
        • PR feat(api): add GitHub webhook receiver for evented refresh #1317
          • PR feat(api): coalesce webhook-driven syncs per repo #1318
            • PR feat(api): add manual sync endpoint for on-demand refresh #1319
              • PR feat(server): default sync interval to 5m and document evented refresh #1320
                • PR feat(server): index repos by owner/repo for GitHub-style routes #1322
                  • PR feat(server): serve repos at /repos/{owner}/{repo} path routes #1323 👈
                  • PR feat(web): GitHub-style path URLs for repos, branches, stacks, PRs #1324

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Merged via consolidation into #1326 by jonnii