Store cook log DB in writable data dir, not read-only recipes mount

The cook log SQLite DB was created at recipeDir/cook-log.db. In the Docker
deployment /recipes is mounted read-only, so SQLite could never create the
file there — the cook log has never worked in the container. The writable
/data volume was mounted but unused.

- initDb now creates the DB's parent directory if missing, so startup
  initialization is self-contained and idempotent.
- app takes a dataDir and places cook-log.db there.
- Add optional --data-dir flag (defaults to the recipe dir, preserving
  local-dev behavior); Docker CMD passes --data-dir /data.

Verified end-to-end as root against a read-only recipe dir and a
non-existent data dir: the dir and DB are created and cook-log POST/GET
round-trips.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Roux/Server.hs

@@ -74,12 +74,15 @@ readRequestBody req = do
             then pure (reverse acc)
             else gather (chunk : acc)
 
-{- | Build the WAI 'Application' given a path to the recipe directory and config.
+{- | Build the WAI 'Application' given the recipe directory, a writable data
+directory, and config.
 
-Recipes are scanned at startup and then watched live via fsnotify.
+Recipes are scanned at startup and then watched live via fsnotify. The cook log
+database is created (if absent) in @dataDir@, which must be writable — the
+recipe directory may be mounted read-only (as it is in the Docker deployment).
 -}
-app :: FilePath -> RouxConfig -> IO Wai.Application
-app recipeDir config = do
+app :: FilePath -> FilePath -> RouxConfig -> IO Wai.Application
+app recipeDir dataDir config = do
     putStrLn $ "[roux] scanning recipes from " <> recipeDir
     recipes <- Idx.scanRecipes recipeDir
     let count = length recipes
@@ -101,8 +104,9 @@ app recipeDir config = do
         forkIO $
             watchRecipes absDir state changeLog `catch` \e ->
                 putStrLn $ "[roux] watcher thread crashed: " <> show (e :: SomeException)
-    -- Initialize cook log database
-    let dbPath = recipeDir </> "cook-log.db"
+    -- Initialize cook log database in the writable data directory (initDb
+    -- creates the directory and file if they are not already present).
+    let dbPath = dataDir </> "cook-log.db"
     db <- CookLog.initDb dbPath
     putStrLn $ "[roux] cook log DB: " <> dbPath
     pure (router absDir config state changeLog db)