feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/backend/internal/runner/helpers.go

@@ -0,0 +1,21 @@
+package runner
+
+import (
+	"regexp"
+	"strings"
+)
+
+// stripMarkdown removes common markdown syntax from src, returning plain text
+// suitable for TTS. Mirrors the helper in the scraper's server package.
+func stripMarkdown(src string) string {
+	src = regexp.MustCompile(`(?m)^#{1,6}\s+`).ReplaceAllString(src, "")
+	src = regexp.MustCompile(`\*{1,3}|_{1,3}`).ReplaceAllString(src, "")
+	src = regexp.MustCompile("(?s)```.*?```").ReplaceAllString(src, "")
+	src = regexp.MustCompile("`[^`]*`").ReplaceAllString(src, "")
+	src = regexp.MustCompile(`\[([^\]]+)\]\([^)]+\)`).ReplaceAllString(src, "$1")
+	src = regexp.MustCompile(`!\[[^\]]*\]\([^)]+\)`).ReplaceAllString(src, "")
+	src = regexp.MustCompile(`(?m)^>\s?`).ReplaceAllString(src, "")
+	src = regexp.MustCompile(`(?m)^[-*_]{3,}\s*$`).ReplaceAllString(src, "")
+	src = regexp.MustCompile(`\n{3,}`).ReplaceAllString(src, "\n\n")
+	return strings.TrimSpace(src)
+}