fix: set timeouts on the upstream HTTP client (#83)

Fixes #67

## Why
The proxy used `http.DefaultClient` for all upstream GET/HEAD and bearer-token requests. It has no timeouts, so a slow or hung upstream holds a goroutine and connection indefinitely.

## Changes
- Add a shared `upstreamClient` (`internal/proxy/httpclient.go`) with dial, TLS-handshake, response-header and idle-connection timeouts, plus connection pooling.
- Deliberately no overall `Client.Timeout`, so large artifact bodies can still stream; total time is bounded by the request context.
- Route all four upstream calls in the engine through it.

## Validation
- `make e2e` passes.

Reviewed-on: #83
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

pkg/models/remote.go

@@ -47,6 +47,11 @@ type Remote struct {
 	MutableTTL   int  `json:"mutable_ttl"`
 	CheckMutable bool `json:"check_mutable"`
 
+	// Upstream HTTP timeouts in seconds. 0 means use the server default.
+	UpstreamDialTimeout           int `json:"upstream_dial_timeout,omitempty"`
+	UpstreamTLSTimeout            int `json:"upstream_tls_timeout,omitempty"`
+	UpstreamResponseHeaderTimeout int `json:"upstream_response_header_timeout,omitempty"`
+
 	Patterns          []string `json:"patterns,omitempty"`
 	Blocklist         []string `json:"blocklist,omitempty"`
 	MutablePatterns   []string `json:"mutable_patterns,omitempty"`