chore: initial commit of Go API gateway

Baseline for the Kong replacement on api.riotpiao.com. Brings the working
tree under version control for the first time: gateway source, the task
board that drives the agent runs, test fixtures, and K8s manifests.

Anchor the gateway ignore rule to the repo root. Unanchored, "gateway"
also matched the cmd/gateway/ source directory, so the program entrypoint
was excluded from every commit.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

internal/config/config.go

@@ -0,0 +1,115 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+// Config holds the gateway configuration.
+type Config struct {
+	// ListenAddr is the address to listen on for HTTP traffic.
+	ListenAddr string
+	// ShutdownTimeout is the maximum time to wait for in-flight requests
+	// to complete before forcing shutdown.
+	ShutdownTimeout time.Duration
+	// Routes maps route names to their upstream configuration.
+	Routes map[string]*Route
+	// Models maps model names to their upstream configuration.
+	// Multiple models can point to the same upstream address.
+	Models map[string]*ModelUpstream
+}
+
+// ModelUpstream holds upstream configuration for a specific model.
+type ModelUpstream struct {
+	// Name is the model name clients send (e.g., "reasoning", "ornith:35b").
+	Name string
+	// Address is the upstream server address (host:port).
+	Address string
+	// Path is the upstream path for this model (e.g., "/v1/chat/completions").
+	Path string
+}
+
+// Route represents a single route and its upstream configuration.
+type Route struct {
+	// Name is the route identifier.
+	Name string
+	// Upstream holds the upstream server configuration.
+	Upstream Upstream
+}
+
+// Upstream holds upstream server configuration for a route.
+type Upstream struct {
+	// Address is the upstream server address (host:port).
+	Address string
+	// PathRewrite is an optional path prefix rewrite. Empty string means no rewrite.
+	PathRewrite string
+	// ConnectTimeout is the maximum time to establish a connection to the upstream.
+	ConnectTimeout time.Duration
+	// ReadTimeout is the maximum time to read a response from the upstream.
+	ReadTimeout time.Duration
+	// WriteTimeout is the maximum time to write a request to the upstream.
+	WriteTimeout time.Duration
+	// MaxBodySize is the maximum request body size in bytes.
+	MaxBodySize int64
+	// AuthRequired indicates whether this route requires authentication.
+	AuthRequired bool
+}
+
+// LookupModel finds a model by name (case-sensitive, exact match).
+func (c *Config) LookupModel(name string) *ModelUpstream {
+	if c == nil || c.Models == nil {
+		return nil
+	}
+	return c.Models[name]
+}
+
+// ModelNames returns a sorted list of all known model names.
+func (c *Config) ModelNames() []string {
+	if c == nil || c.Models == nil {
+		return nil
+	}
+	names := make([]string, 0, len(c.Models))
+	for name := range c.Models {
+		names = append(names, name)
+	}
+	return names
+}
+
+// Load reads configuration from environment variables with defaults.
+func Load() (*Config, error) {
+	listenAddr := "127.0.0.1:8080"
+	// Allow override via environment variable
+	if addr, ok := os.LookupEnv("LISTEN_ADDR"); ok {
+		listenAddr = addr
+	}
+
+	shutdownTimeout := 30 * time.Second
+	// Allow override via environment variable
+	if timeout, ok := os.LookupEnv("SHUTDOWN_TIMEOUT"); ok {
+		d, err := time.ParseDuration(timeout)
+		if err != nil {
+			return nil, fmt.Errorf("invalid SHUTDOWN_TIMEOUT: %w", err)
+		}
+		shutdownTimeout = d
+	}
+
+	// Load routes and models from config file
+	routes := make(map[string]*Route)
+	models := make(map[string]*ModelUpstream)
+	if configPath, ok := os.LookupEnv("CONFIG_PATH"); ok {
+		loadedRoutes, loadedModels, err := LoadRoutesAndModelsFromFile(configPath)
+		if err != nil {
+			return nil, err
+		}
+		routes = loadedRoutes
+		models = loadedModels
+	}
+
+	return &Config{
+		ListenAddr:      listenAddr,
+		ShutdownTimeout: shutdownTimeout,
+		Routes:          routes,
+		Models:          models,
+	}, nil
+}