chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

backend/internal/browser/browser.go

@@ -1,7 +1,4 @@
 // Package browser provides a rate-limited HTTP client for web scraping.
-// The Client interface is the only thing the rest of the codebase depends on;
-// the concrete DirectClient can be swapped for any other implementation
-// (e.g. a Browserless-backed client) without touching callers.
 package browser
 
 import (
@@ -10,7 +7,6 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"net/url"
 	"strconv"
 	"sync"
 	"time"
@@ -51,9 +47,6 @@ type Config struct {
 	MaxConcurrent int
 	// Timeout is the per-request deadline. Defaults to 90s when 0.
 	Timeout time.Duration
-	// ProxyURL is an optional outbound proxy, e.g. "http://user:pass@host:3128".
-	// Falls back to HTTP_PROXY / HTTPS_PROXY environment variables when empty.
-	ProxyURL string
 }
 
 // DirectClient is a plain net/http-based Client with a concurrency semaphore.
@@ -75,14 +68,6 @@ func NewDirectClient(cfg Config) *DirectClient {
 		MaxIdleConnsPerHost: cfg.MaxConcurrent * 2,
 		DisableCompression:  false,
 	}
-	if cfg.ProxyURL != "" {
-		proxyParsed, err := url.Parse(cfg.ProxyURL)
-		if err == nil {
-			transport.Proxy = http.ProxyURL(proxyParsed)
-		}
-	} else {
-		transport.Proxy = http.ProxyFromEnvironment
-	}
 
 	return &DirectClient{
 		http: &http.Client{