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/docs/architecture.mermaid.md

@@ -0,0 +1,59 @@
+```mermaid
+graph LR
+    %% ── External ──────────────────────────────────────────────────────────
+    NF([novelfire.net])
+    KK([Kokoro-FastAPI TTS])
+    LE([Let's Encrypt])
+    CL([Browser / iOS App])
+
+    %% ── Init containers ───────────────────────────────────────────────────
+    subgraph INIT["Init containers (one-shot)"]
+        MI[minio-init\nmc: create buckets]
+        PI[pb-init\nbootstrap collections]
+    end
+
+    %% ── Storage ───────────────────────────────────────────────────────────
+    subgraph STORAGE["Storage"]
+        MN[(MinIO :9000\nchapters · audio\navatars · browse)]
+        PB[(PocketBase :8090\nbooks · chapters_idx\naudio_cache · progress\nscrape_jobs · app_users · ranking)]
+        VK[(Valkey :6379\npresign URL cache\nTTL-based · shared)]
+        MS[(Meilisearch :7700\nindex: books)]
+    end
+
+    %% ── Application ───────────────────────────────────────────────────────
+    subgraph APP["Application"]
+        CD[Caddy :443/:80\nreverse proxy\nauto-HTTPS]
+        BE[Backend API :8080\nGo HTTP server]
+        RN[Runner :9091\nGo background worker\n/metrics endpoint]
+        UI[SvelteKit UI :3000\nadapter-node]
+    end
+
+    %% ── Init → Storage ────────────────────────────────────────────────────
+    MI -.->|create buckets| MN
+    PI -.->|bootstrap schema| PB
+
+    %% ── App → Storage ─────────────────────────────────────────────────────
+    BE -->|blobs| MN
+    BE -->|structured records| PB
+    BE -->|cache presigned URLs| VK
+    RN -->|chapter markdown & audio| MN
+    RN -->|read/update jobs & books| PB
+    RN -->|index books on scrape| MS
+    UI -->|read presign cache| VK
+
+    %% ── App internal ──────────────────────────────────────────────────────
+    UI -->|REST API| BE
+
+    %% ── Caddy routing ─────────────────────────────────────────────────────
+    CD -->|/* proxy| UI
+    CD -->|/api/* /health proxy| BE
+    CD -->|/s3/* proxy internal| MN
+
+    %% ── Runner → External ─────────────────────────────────────────────────
+    RN -->|scrape HTTP GET| NF
+    RN -->|TTS HTTP POST| KK
+    CD -->|ACME certificate| LE
+
+    %% ── Client ────────────────────────────────────────────────────────────
+    CL -->|HTTPS :443 single entry| CD
+```