feat: manga bookmark sync backend + Bromite userscript

Backend (Go, stdlib net/http + modernc.org/sqlite, CGO-free static binary):
- GET/PUT/DELETE /bookmarks{,/key} + /healthz
- bearer auth (constant-time), CORS origin reflection + 204 preflight
- SQLite store keyed <site>:<series_id>, last-write-wins, server-set updated_at
- httptest + temp-sqlite tests (auth, CORS, round-trip); go vet clean
- multi-stage Dockerfile (distroless static nonroot) + compose (base + prod proxy override)

Userscript (single Bromite-compatible IIFE, no GM_* APIs):
- Asura + Demonic adapters, URL-regex ids + og: title/cover
- Shadow-DOM floating UI, localStorage cache, optimistic sync
- auto-progress (no regress) + manual override; framework-agnostic nav watcher

Live-verified adapters (Playwright, 2026-07-24): asurascans.com /comics/<slug-hash>,
demonicscans.org /manga/<slug> + /title/<slug>/chapter/<n> — corrects the plan's
assumed /series/ paths and asuracomic.net domain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/handlers.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type bookmarkHandler struct {
+	store *Store
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if v != nil {
+		if err := json.NewEncoder(w).Encode(v); err != nil {
+			log.Printf("encode response: %v", err)
+		}
+	}
+}
+
+// list returns all bookmarks. GET /bookmarks
+func (h *bookmarkHandler) list(w http.ResponseWriter, r *http.Request) {
+	items, err := h.store.List()
+	if err != nil {
+		log.Printf("list: %v", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	writeJSON(w, http.StatusOK, items)
+}
+
+// put upserts one bookmark. PUT /bookmarks/{key}
+func (h *bookmarkHandler) put(w http.ResponseWriter, r *http.Request) {
+	key := r.PathValue("key")
+	if key == "" {
+		http.Error(w, "missing key", http.StatusBadRequest)
+		return
+	}
+
+	var b Bookmark
+	if err := json.NewDecoder(http.MaxBytesReader(w, r.Body, 1<<16)).Decode(&b); err != nil {
+		http.Error(w, "invalid JSON body", http.StatusBadRequest)
+		return
+	}
+
+	// Path key is authoritative; derive site/series_id from it when the body
+	// omits them so the stored row is always self-consistent.
+	b.Key = key
+	if b.Site == "" || b.SeriesID == "" {
+		if site, series, ok := strings.Cut(key, ":"); ok {
+			if b.Site == "" {
+				b.Site = site
+			}
+			if b.SeriesID == "" {
+				b.SeriesID = series
+			}
+		}
+	}
+	b.UpdatedAt = time.Now().UnixMilli() // server-assigned, ignore client value
+
+	if err := h.store.Upsert(b); err != nil {
+		log.Printf("upsert: %v", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	writeJSON(w, http.StatusOK, b)
+}
+
+// delete removes one bookmark. DELETE /bookmarks/{key}
+func (h *bookmarkHandler) delete(w http.ResponseWriter, r *http.Request) {
+	key := r.PathValue("key")
+	if key == "" {
+		http.Error(w, "missing key", http.StatusBadRequest)
+		return
+	}
+	if err := h.store.Delete(key); err != nil {
+		log.Printf("delete: %v", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func healthz(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("ok"))
+}