Initial file-ingestion service

REST endpoint (Oxygen.jl POST /upload, multipart) that spools uploaded
files to disk, enqueues lightweight references onto a bounded thread-safe
work queue, and hands off immediately (202 + job IDs; 503 when full). A
configurable pool of worker threads pulls jobs off the queue, logs the
received filename (placeholder for real processing), and moves files to
done/ on success or failed/ on error.

- Queue behind an enqueue!/dequeue!/close! seam for a future RabbitMQ swap
- Startup recovery: re-enqueues leftover files in spool/
- Graceful drain on SIGINT and SIGTERM (via atexit)
- Env-var config; filenames sanitized + UUID-prefixed on disk

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

src/config.jl

@@ -0,0 +1,46 @@
+# Runtime configuration. Built once at startup, then treated as immutable.
+# Every knob has a default so the service runs with zero configuration, and
+# every knob can be overridden by an environment variable (see `config_from_env`).
+
+Base.@kwdef struct Config
+    host::String        = "127.0.0.1"
+    port::Int           = 8080
+    worker_count::Int   = Threads.nthreads()
+    queue_capacity::Int = 1000
+    spool_dir::String   = "data/spool"   # files land here on intake (pending)
+    done_dir::String    = "data/done"    # files move here after successful processing
+    failed_dir::String  = "data/failed"  # files move here if a worker throws
+end
+
+"""
+    config_from_env(; overrides...)
+
+Build a `Config` from environment variables, falling back to the struct
+defaults. Any keyword `overrides` win over the environment (useful for tests
+and for `FileServer.run(; port=...)`).
+
+Recognised variables:
+  FS_HOST, FS_PORT, FS_WORKERS, FS_QUEUE_CAPACITY,
+  FS_SPOOL_DIR, FS_DONE_DIR, FS_FAILED_DIR
+"""
+function config_from_env(; host=nothing, port=nothing, worker_count=nothing,
+                          queue_capacity=nothing, spool_dir=nothing,
+                          done_dir=nothing, failed_dir=nothing)
+    Config(
+        host           = something(host,           get(ENV, "FS_HOST", "127.0.0.1")),
+        port           = something(port,           parse(Int, get(ENV, "FS_PORT", "8080"))),
+        worker_count   = something(worker_count,   parse(Int, get(ENV, "FS_WORKERS", string(Threads.nthreads())))),
+        queue_capacity = something(queue_capacity, parse(Int, get(ENV, "FS_QUEUE_CAPACITY", "1000"))),
+        spool_dir      = something(spool_dir,      get(ENV, "FS_SPOOL_DIR", "data/spool")),
+        done_dir       = something(done_dir,       get(ENV, "FS_DONE_DIR", "data/done")),
+        failed_dir     = something(failed_dir,     get(ENV, "FS_FAILED_DIR", "data/failed")),
+    )
+end
+
+"Create the spool/done/failed directories if they don't already exist."
+function ensure_dirs(cfg::Config)
+    for d in (cfg.spool_dir, cfg.done_dir, cfg.failed_dir)
+        mkpath(d)
+    end
+    return nothing
+end