feat(core): implement file-based API versioning system (DAW/furt#83)

- Add VERSION file in repository root
- Add read_version() function with error handling
- Update /health endpoint to show file-based version
- Add version display during server startup
- Fallback to ?.?.? when VERSION file unreadable

Enables deployment tracking across dev/test/prod environments

VERSION

@@ -0,0 +1 @@
+0.1.1

src/main.lua

@@ -13,6 +13,25 @@ local AuthRoute = require("src.routes.auth")
 -- Load configuration
 local config = require("config.server")
 
+-- Read version from VERSION file
+local function read_version()
+    local file, err = io.open("VERSION", "r")
+    if not file then
+        print("WARNING: Could not read VERSION file: " .. (err or "unknown error"))
+        return "?.?.?"
+    end
+
+    local version = file:read("*line")
+    file:close()
+
+    if not version or version:match("^%s*$") then
+        print("WARNING: VERSION file is empty or contains only whitespace")
+        return "?.?.?"
+    end
+
+    return version:match("^%s*(.-)%s*$") -- trim whitespace
+end
+
 -- HTTP-Server Module
 local FurtServer = {}
 
@@ -47,23 +66,23 @@ function FurtServer:parse_request(client)
     if not request_line then
         return nil
     end
-    
+
     -- Parse request line: "POST /v1/mail/send HTTP/1.1"
     local method, path, protocol = request_line:match("(%w+) (%S+) (%S+)")
     if not method then
         return nil
     end
-    
+
     -- Parse headers
     local headers = {}
     local content_length = 0
-    
+
     while true do
         local line = client:receive()
         if not line or line == "" then
             break
         end
-        
+
         local key, value = line:match("([^:]+): (.+)")
         if key and value then
             headers[key:lower()] = value
@@ -72,13 +91,13 @@ function FurtServer:parse_request(client)
             end
         end
     end
-    
+
     -- Parse body
     local body = ""
     if content_length > 0 then
         body = client:receive(content_length)
     end
-    
+
     return {
         method = method,
         path = path,
@@ -97,11 +116,11 @@ function FurtServer:add_cors_headers(request)
         "https://dragons-at-work.de",
         "https://www.dragons-at-work.de"
     }
-    
+
     -- Check if request has Origin header
     local origin = request and request.headers and request.headers.origin
     local cors_origin = "*"  -- Default fallback
-    
+
     -- If origin is provided and in allowed list, use it
     if origin then
         for _, allowed in ipairs(allowed_origins) do
@@ -111,7 +130,7 @@ function FurtServer:add_cors_headers(request)
             end
         end
     end
-    
+
     return {
         ["Access-Control-Allow-Origin"] = cors_origin,
         ["Access-Control-Allow-Methods"] = "GET, POST, PUT, DELETE, OPTIONS",
@@ -125,38 +144,38 @@ end
 function FurtServer:create_response(status, data, content_type, additional_headers, request)
     content_type = content_type or "application/json"
     local body = ""
-    
+
     if type(data) == "table" then
         body = cjson.encode(data)
     else
         body = tostring(data or "")
     end
-    
+
     -- Start with CORS headers
     local headers = self:add_cors_headers(request)
-    
+
     -- Add standard headers
     headers["Content-Type"] = content_type
     headers["Content-Length"] = tostring(#body)
     headers["Connection"] = "close"
     headers["Server"] = "Furt-Lua/1.1"
-    
+
     -- Add additional headers if provided
     if additional_headers then
         for key, value in pairs(additional_headers) do
             headers[key] = value
         end
     end
-    
+
     -- Build response
     local response = string.format("HTTP/1.1 %d %s\r\n", status, self:get_status_text(status))
-    
+
     for key, value in pairs(headers) do
         response = response .. key .. ": " .. value .. "\r\n"
     end
-    
+
     response = response .. "\r\n" .. body
-    
+
     return response
 end
 
@@ -184,23 +203,23 @@ function FurtServer:handle_client(client)
         client:send(response)
         return
     end
-    
+
-    print(string.format("[%s] %s %s", os.date("%Y-%m-%d %H:%M:%S"), 
+    print(string.format("[%s] %s %s", os.date("%Y-%m-%d %H:%M:%S"),
           request.method, request.path))
-    
+
     -- Handle OPTIONS preflight requests (CORS)
     if request.method == "OPTIONS" then
         local response = self:create_response(204, "", "text/plain", nil, request)
         client:send(response)
         return
     end
-    
+
     -- Route handling
     local handler = nil
     if self.routes[request.method] and self.routes[request.method][request.path] then
         handler = self.routes[request.method][request.path]
     end
-    
+
     if handler then
         local success, result = pcall(handler, request, self)
         if success then
@@ -223,13 +242,16 @@ function FurtServer:start()
     if not self.server then
         error("Failed to bind to " .. self.host .. ":" .. self.port)
     end
-    
+
+    local version = read_version()
+
     print(string.format("Furt HTTP-Server started on %s:%d", self.host, self.port))
+    print("Version: " .. version)
     print("API-Key authentication: ENABLED")
     print("Rate limiting: ENABLED (60 req/hour per API key, 100 req/hour per IP)")
     print("CORS enabled for configured origins")
     print("Press Ctrl+C to stop")
-    
+
     while true do
         local client = self.server:accept()
         if client then
@@ -245,10 +267,11 @@ local server = FurtServer:new()
 
 -- Public routes (no authentication required)
 server:add_route("GET", "/health", function(request, server)
+    local version = read_version()
     local response_data = {
         status = "healthy",
         service = "furt-lua",
-        version = "1.1.0",
+        version = version,
         timestamp = os.time(),
         features = {
             smtp_configured = config.mail and config.mail.username ~= nil,
@@ -268,15 +291,15 @@ if os.getenv("ENABLE_TEST_ENDPOINT") == "true" then
             headers_count = 0,
             warning = "This is a development endpoint"
         }
-        
+
         -- Count headers
         for _ in pairs(request.headers) do
             response_data.headers_count = response_data.headers_count + 1
         end
-        
+
         return server:create_response(200, response_data, nil, nil, request)
     end)
-    print("⚠️  Test endpoint enabled (development mode)")
+    print("[WARN] Test endpoint enabled (development mode)")
 end
 
 -- Protected routes (require authentication)