feat: add install script and fix Docker environment config

- Add install.sh for one-line installation (Linux/macOS)
  - Detects local Ollama and lets user choose system vs Docker
  - Generates docker-compose.override.yml for system Ollama mode
  - Supports --update and --uninstall flags
- Fix backend not reading OLLAMA_URL from environment variable
  - Add getEnvOrDefault() helper for PORT, DB_PATH, OLLAMA_URL
  - Update Dockerfile to use env vars instead of hardcoded flags
- Update README with new Quick Start instructions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/cmd/server/main.go

@@ -17,11 +17,18 @@ import (
 	"vessel-backend/internal/database"
 )
 
+func getEnvOrDefault(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}
+
 func main() {
 	var (
-		port      = flag.String("port", "8080", "Server port")
-		dbPath    = flag.String("db", "./data/vessel.db", "Database file path")
-		ollamaURL = flag.String("ollama-url", "http://localhost:11434", "Ollama API URL")
+		port      = flag.String("port", getEnvOrDefault("PORT", "8080"), "Server port")
+		dbPath    = flag.String("db", getEnvOrDefault("DB_PATH", "./data/vessel.db"), "Database file path")
+		ollamaURL = flag.String("ollama-url", getEnvOrDefault("OLLAMA_URL", "http://localhost:11434"), "Ollama API URL")
 	)
 	flag.Parse()