feat(gitea): surface mathias's own open Gitea issues on stage 03 (#4)

TDD: IssueNodes parses /repos/issues/search into stage nodes (title, repo
tag, clickable html_url). gitea.MyIssues() reads GITEA_TOKEN and skips
gracefully when unset, mirroring the existing liveOverlay fallback pattern.

Single-operator homelab, not per-visitor OAuth: a static read-only PAT
gates on "cleared Authentik forward-auth", not per-user token exchange —
see #4 discussion. GITEA_TOKEN provisioning in infra (ExternalSecret) is
a separate follow-up; without it the overlay is inert (no crash, just no
live nodes), so this ships safely ahead of that wiring.

Nodes with a url now render as clickable <a class="node"> instead of
<div class="node">.

internal/atlas/model.go

@@ -23,6 +23,7 @@ type Node struct {
 	Tags       []string `json:"tags,omitempty"`
 	Risk       bool     `json:"risk,omitempty"`
 	Gate       bool     `json:"gate,omitempty"`
+	URL        string   `json:"url,omitempty"`
 }
 
 // Stage is one column of the pipeline. PlainTitle/Plain are the plain-language