config,server: add upstream.ignorePaths (#869)

Add upstream.ignorePaths config to prevent model swaps for static-asset
requests made through the /upstream/<model>/<path> passthrough endpoint.

- add UpstreamConfig with compiled *regexp.Regexp slice; invalid regex
returns an error at load time
- apply a default pattern matching common static-asset suffixes
(.js/.json/.css/.png/.gif/.jpg/.jpeg/.ico/.txt) when unset
- in handleUpstream, return 409 Conflict when a path matches and the
local model is not already loaded; peer and already-loaded models fall
through to normal dispatch
- update config-schema.json and config.example.yaml

Updates discussion: #868

internal/config/config.go

@@ -163,6 +163,9 @@ type Config struct {
 
 	// support remote peers, see issue #433, #296
 	Peers PeerDictionaryConfig `yaml:"peers"`
+
+	// upstream controls behaviour of the /upstream passthrough endpoint
+	Upstream UpstreamConfig `yaml:"upstream"`
 }
 
 // RoutingConfig is the canonical, normalized routing/scheduling configuration.
@@ -270,6 +273,12 @@ func LoadConfigFromReader(r io.Reader) (Config, error) {
 		return Config{}, fmt.Errorf("globalTTL must be >= 0")
 	}
 
+	// Apply default for upstream.ignorePaths when not specified. The default
+	// matches common static-asset suffixes so they do not trigger a swap.
+	if len(config.Upstream.IgnorePaths) == 0 {
+		config.Upstream.IgnorePaths = DefaultUpstreamIgnorePaths()
+	}
+
 	switch config.LogToStdout {
 	case LogToStdoutProxy, LogToStdoutUpstream, LogToStdoutBoth, LogToStdoutNone:
 	default: