Persist config files across Opus package updates

The Opus package manager deletes the entire package directory on
update (fs.delete(packageDir)) before re-downloading files. This
wiped all config files (.manager_config, .client_config, etc.) that
were stored inside packages/inventory-manager/.

Fix: add _configPath() helper to every program that resolves config
file paths to usr/config/inventory-manager/ when running under Opus,
which lives outside the package directory and survives updates.
Falls back to the local _path() for standalone (non-Opus) use.

Updated files:
- inventoryManager.lua, manager/config.lua, inventoryClient.lua,
  inventoryWebBridge.lua, dropperController.lua, craftingTurtle.lua
- .package install script: saves configs to usr/config/inventory-manager/
- autorun/startup.lua: checks both persistent and package dirs
- startup/client.lua: uses persistent dir for .client_setup/.dropper_config
- web/server/Dockerfile: switch health check to wget (from prior fix)

autorun/startup.lua

@@ -8,17 +8,23 @@ local peripheral = _G.peripheral
 local shell      = _ENV.shell
 
 local BASE = 'packages/inventory-manager'
+local CFG  = 'usr/config/inventory-manager'
 
 -------------------------------------------------
 -- Determine role from config files written during install
 -------------------------------------------------
 
+local function cfgExists(name)
+    return fs.exists(fs.combine(CFG, name))
+        or fs.exists(fs.combine(BASE, name))
+end
+
 local role
-if fs.exists(fs.combine(BASE, '.manager_config')) then
+if cfgExists('.manager_config') then
     role = 'manager'
-elseif fs.exists(fs.combine(BASE, '.client_config')) then
+elseif cfgExists('.client_config') then
     role = 'client'
-elseif fs.exists(fs.combine(BASE, '.webbridge_config')) then
+elseif cfgExists('.webbridge_config') then
     role = 'bridge'
 elseif _G.turtle then
     role = 'turtle'