stop(logging): implement structured JSON logging then halt due to YAGNI

Add comprehensive structured logging system, then stop development:

- Create Logger module with JSON output and configurable log levels
- Implement hash-based request ID generation for request tracing
- Add performance timing and client IP detection to HTTP server
- Enhance startup logging with module loading and configuration checks

STOPPED: Feature violates Low-Tech principles
- 200+ lines logging vs 100 lines business logic (code bloat)
- JSON serialization overhead reduces performance
- No current production need for structured monitoring
- Simple print() statements sufficient for current scale

Branch parked for future consideration when monitoring requirements
actually emerge. Issue #54 deferred to v0.2.x milestone.

src/logger.lua

@@ -0,0 +1,200 @@
+-- src/logger.lua
+-- Structured JSON Logger for Furt API-Gateway
+-- Dragons@Work Digital Sovereignty Project
+
+local found_cjson, cjson = pcall(require, 'cjson')
+if not found_cjson then
+    cjson = require('dkjson')
+end
+
+local config = require("config.server")
+
+-- Hash-based request ID generator for collision resistance
+local function generate_request_id()
+    local data = string.format("%d-%d-%d-%d",
+        os.time(),
+        math.random(1000000, 9999999),
+        math.random(1000000, 9999999),
+        os.clock() * 1000000)
+
+    -- Simple hash function (Lua-native, no dependencies)
+    local hash = 0
+    for i = 1, #data do
+        hash = (hash * 31 + string.byte(data, i)) % 2147483647
+    end
+
+    return string.format("req-%x", hash)
+end
+
+-- Export request ID generator
+Logger = {}
+Logger.generate_request_id = generate_request_id
+
+local Logger = {}
+
+-- Log levels with numeric values for filtering
+local LOG_LEVELS = {
+    debug = 1,
+    info = 2,
+    warn = 3,
+    error = 4
+}
+
+-- Current log level from config
+local current_log_level = LOG_LEVELS[config.log_level] or LOG_LEVELS.info
+
+-- Service identification
+local SERVICE_NAME = "furt-lua"
+
+-- Generate timestamp in ISO format
+local function get_timestamp()
+    return os.date("!%Y-%m-%dT%H:%M:%SZ")
+end
+
+-- Core logging function
+local function log_structured(level, message, context)
+    -- Skip if log level is below threshold
+    if LOG_LEVELS[level] < current_log_level then
+        return
+    end
+
+    -- Build log entry
+    local log_entry = {
+        timestamp = get_timestamp(),
+        level = level,
+        service = SERVICE_NAME,
+        message = message
+    }
+
+    -- Add context data if provided
+    if context then
+        for key, value in pairs(context) do
+            log_entry[key] = value
+        end
+    end
+
+    -- Output as JSON
+    local json_output = cjson.encode(log_entry)
+    print(json_output)
+end
+
+-- Public logging functions
+function Logger.debug(message, context)
+    log_structured("debug", message, context)
+end
+
+function Logger.info(message, context)
+    log_structured("info", message, context)
+end
+
+function Logger.warn(message, context)
+    log_structured("warn", message, context)
+end
+
+function Logger.error(message, context)
+    log_structured("error", message, context)
+end
+
+-- Request logging with performance metrics
+function Logger.log_request(method, path, status, duration_ms, client_ip, request_id)
+    if not config.log_requests then
+        return
+    end
+
+    local context = {
+        method = method,
+        path = path,
+        status = status,
+        duration_ms = duration_ms,
+        client_ip = client_ip
+    }
+
+    -- Add request_id if provided
+    if request_id then
+        context.request_id = request_id
+    end
+
+    log_structured("info", "HTTP request", context)
+end
+
+-- Service startup logging
+function Logger.log_startup(host, port, version_info)
+    Logger.info("Furt HTTP-Server starting", {
+        host = host,
+        port = port,
+        version = version_info.version,
+        content_hash = version_info.content_hash,
+        vcs_hash = version_info.vcs_info and version_info.vcs_info.hash
+    })
+end
+
+-- Service health logging
+function Logger.log_health_check(status, details)
+    local level = status == "healthy" and "info" or "warn"
+    log_structured(level, "Health check", {
+        health_status = status,
+        details = details
+    })
+end
+
+-- Error logging with stack trace support
+function Logger.log_error(error_message, context, stack_trace)
+    local error_context = context or {}
+    if stack_trace then
+        error_context.stack_trace = stack_trace
+    end
+    log_structured("error", error_message, error_context)
+end
+
+-- Configuration logging
+function Logger.log_config_summary(summary)
+    Logger.info("Configuration loaded", summary)
+end
+
+-- Rate limiting events
+function Logger.log_rate_limit(api_key, client_ip, limit_type)
+    Logger.warn("Rate limit exceeded", {
+        api_key = api_key and "***masked***" or nil,
+        client_ip = client_ip,
+        limit_type = limit_type
+    })
+end
+
+-- SMTP/Mail logging
+function Logger.log_mail_event(event_type, recipient, success, error_message)
+    local level = success and "info" or "error"
+    log_structured(level, "Mail event", {
+        event_type = event_type,
+        recipient = recipient,
+        success = success,
+        error = error_message
+    })
+end
+
+-- Set log level dynamically (useful for debugging)
+function Logger.set_log_level(level)
+    if LOG_LEVELS[level] then
+        current_log_level = LOG_LEVELS[level]
+        Logger.info("Log level changed", { new_level = level })
+    else
+        Logger.error("Invalid log level", { attempted_level = level })
+    end
+end
+
+-- Get current log level
+function Logger.get_log_level()
+    for level, value in pairs(LOG_LEVELS) do
+        if value == current_log_level then
+            return level
+        end
+    end
+    return "unknown"
+end
+
+-- Check if a log level would be output
+function Logger.would_log(level)
+    return LOG_LEVELS[level] and LOG_LEVELS[level] >= current_log_level
+end
+
+return Logger
+