Split backend into internal packages by responsibility

All Go files lived flat in backend/ as one package main. Move store,
latest-chapter polling, sessions, HTTP middleware, the JSON API, the
userscript handler, and the web UI (with its templates/static assets)
into backend/internal/{store,latest,session,httpmw,api,userscript,web},
each with an exported API. main.go becomes the composition root wiring
them into newRouter; root-level tests cover the assembled router while
package-local tests cover unit behavior. Update Dockerfile/.dockerignore
for the new internal/ tree and CLAUDE.md to describe the layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/api/handlers.go

@@ -0,0 +1,116 @@
+package api
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+
+	"mangabm/backend/internal/store"
+)
+
+// Handler serves the userscript-facing JSON bookmark API.
+type Handler struct {
+	Store *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 *Handler) 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 *Handler) Put(w http.ResponseWriter, r *http.Request) {
+	key := r.PathValue("key")
+	if key == "" {
+		http.Error(w, "missing key", http.StatusBadRequest)
+		return
+	}
+
+	var b store.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
+			}
+		}
+	}
+
+	// An empty status is "no opinion" and Upsert keeps the stored bucket.
+	// Finishing a series is a web-UI decision, so the JSON API refuses it
+	// rather than trusting every client to leave it alone.
+	switch b.Status {
+	case "", store.StatusReading, store.StatusArchived:
+	case store.StatusFinished:
+		http.Error(w, "status "+store.StatusFinished+" can only be set from the web UI",
+			http.StatusBadRequest)
+		return
+	default:
+		http.Error(w, "invalid status", http.StatusBadRequest)
+		return
+	}
+
+	// Candidate timestamp, not a decision: Upsert keeps the stored one unless
+	// reading progress actually moved. Any client value is ignored.
+	b.UpdatedAt = time.Now().UnixMilli()
+
+	stored, err := h.Store.Upsert(b)
+	if err != nil {
+		log.Printf("upsert: %v", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	// Echo the stored row: clients adopt this as their cached copy, so it must
+	// carry the authoritative updated_at rather than the candidate above.
+	writeJSON(w, http.StatusOK, stored)
+}
+
+// Delete removes one bookmark. DELETE /bookmarks/{key}
+func (h *Handler) 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)
+}
+
+// Healthz answers the unauthenticated liveness check. GET /healthz
+func Healthz(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("ok"))
+}