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>

Project.toml

@@ -0,0 +1,18 @@
+name = "FileServer"
+uuid = "b3f1c2d4-5e6a-4b7c-8d9e-0f1a2b3c4d5e"
+version = "0.1.0"
+authors = ["wardjm@gmail.com"]
+
+[deps]
+HTTP = "cd3eb016-35fb-5094-929b-558a96fad6f3"
+JSON3 = "0f8b85d8-7281-11e9-16c2-39a750bddbf1"
+Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"
+Oxygen = "df9a0d86-3283-4920-82dc-4555fc0d1d8b"
+UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
+
+[compat]
+HTTP = "1.11.0"
+JSON3 = "1.14.3"
+Logging = "1.11.0"
+Oxygen = "1.10.2"
+UUIDs = "1.11.0"