feat: replace Bitwarden CLI with native Go API client (#2)

BREAKING: Removes Node.js/Bitwarden CLI dependency entirely.

## What changed
- New native HTTP client talks directly to Vaultwarden API
- Bitwarden-compatible crypto in pure Go:
  - AES-256-CBC + HMAC-SHA256 decryption
  - PBKDF2-SHA256 and Argon2id key derivation
  - HKDF key stretching (with legacy fallback)
- Auto token refresh (never expires)
- Background vault sync (configurable interval, default 5min)
- Docker image: ~233MB → ~15MB (pure Alpine, no Node.js)

## New env vars
- VAULTWARDEN_EMAIL (replaces CLIENT_ID/CLIENT_SECRET)
- SYNC_INTERVAL (default: 5m)

## Removed
- internal/vaultwarden/client_cli.go (CLI interaction)
- Bitwarden CLI dependency (Node.js + npm package)
- VAULTWARDEN_CLIENT_ID, VAULTWARDEN_CLIENT_SECRET env vars

## Added
- internal/vaultwarden/crypto.go (encryption/decryption)
- internal/vaultwarden/crypto_test.go (11 unit tests)
- internal/vaultwarden/api_client.go (HTTP client)

## Unchanged
- External API surface (GET /secret/:name, POST /refresh, GET /health)
- All security middleware (auth, IP whitelist, rate limiting)

go.mod

@@ -2,11 +2,14 @@ module github.com/thijsherman/vaultwarden-api
 
 go 1.25.3
 
-require github.com/gofiber/fiber/v2 v2.52.9
+require (
+	github.com/gofiber/fiber/v2 v2.52.9
+	github.com/google/uuid v1.6.0
+	golang.org/x/crypto v0.48.0
+)
 
 require (
 	github.com/andybalholm/brotli v1.1.0 // indirect
-	github.com/google/uuid v1.6.0 // indirect
 	github.com/klauspost/compress v1.17.9 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
@@ -17,5 +20,5 @@ require (
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasthttp v1.51.0 // indirect
 	github.com/valyala/tcplisten v1.0.0 // indirect
-	golang.org/x/sys v0.28.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
 )