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/.dockerignore

@@ -0,0 +1,5 @@
+# Only go source + module files are needed in the build context.
+*
+!go.mod
+!go.sum
+!*.go