Add the media downloader with hash storage and local file serving

CURRENT.md

@@ -33,13 +33,15 @@ mobile draft at 390px.
   - [x] C7a storage limits, retention, warning/confirm flow, storage and clean API
   - [x] C7b Settings group: limit field with info hover, usage bar, clean modal
   - [x] C8 MCP service hooks (`scrape_media`, scrape reads, storage, clean; reads in `run_commands`)
-  - [ ] C10 downloader: fetch attachment bytes in `mode: "download"`, hash-dedupe, size cap,
-    serial queue with 1-3 s gaps, retries. The command accepts the mode today but only indexes.
+  - [x] C10 downloader: attachment and sticker bytes, hash-addressed, serial, size-capped, retries;
+    embeds excluded in SQL; local files served at `/api/media/<scrape>/<item>/file` and preferred
+    by the Media tab. Live run 2026-09-16: scrape `...-20260916083401` fetched the 784 KB
+    attachment; storage counted actual bytes.
   - [~] C9 live verification. Slopbot path verified live on 2026-09-16: login Slopbot#5775, guild
-    `177931418704281600`, test channel `1549681689222053988`. Scrape `1549681689222053988-
-    20260916075150`: state done, 1 page, 17 messages, 4 images + 13 videos, 0 errors, ~20 MB of
-    indexed source sizes. Still owed: the puppet `message.fetch` field check, a `download` mode
-    run, a real 429 observation, and pacing tuning.
+    `177931418704281600`, test channel `1549681689222053988`. Links scrape `...-20260916075150`:
+    done, 1 page, 17 messages, 4 images + 13 videos, 0 errors. Download scrape `...-20260916083401`:
+    done, 1 file. Still owed: preview caching (C11), the puppet `message.fetch` field check, a real
+    429 observation, and pacing tuning.
 - Flextalk voice profile, end to end. The profile window is a movable panel that recenters on each
   open; it carries the character sliders with engine defaults and +/-2 snapping, five named
   presets, radio-kit effects (high-pass 40-400 Hz, echo 0-100, word mode with a gap in ms), and