fix: Discord downloads land as None/<date>_None_<name> with no post — name them from the keys gallery-dl really emits

Operator report: a Discord source showed the wrong post time. Listing the real downloads found every Discord folder on the instance (7 artist folders, ~1,600 media) in a directory named `None`, as `<date>_None_<original name>`, next to sidecars named `<original name>.json`.

The real sidecar settles why:
- gallery-dl's discord extractor emits `channel` as a plain string, the message as `message_id`, and the attachment position as `num`. It has no `id` key.
- The patterns asked for `{channel[name]}` and `{id}`. gallery-dl renders a missing field as "None" and carries on.
- The sidecar was named `{filename}.json`, the attachment's ORIGINAL name. find_sidecar never pairs that with `<date>_None_<name>.png`, so no Discord file ever got a Post or a post date. The card fell back to downloaded_at.
- Every `image.png` in a channel also overwrote the same `image.json`.

Fix (gallery_dl.py):
- The directory is `{channel}`.
- The filename is `{date:%Y%m%d}_{message_id}_{num:>02}_{filename}.{extension}`, unique per attachment.
- A discord-scoped metadata postprocessor names the sidecar exactly like the media minus its extension, so find_sidecar's first candidate matches. A per-source filename override re-derives the sidecar name. save_metadata=False drops it.

Guard (tests/test_gallery_dl_naming.py) renders the patterns through Python's formatter against a sanitized copy of the real sidecar (same keys and types, invented values). A missing key or a subscript into a string raises, which is the loud failure gallery-dl doesn't give. A positive control shows both shipped patterns fail it.

Existing broken downloads are NOT repaired by this. gallery-dl's archive already records them, so a re-run skips them, and their collided sidecars no longer describe them. That repair is a separate, destructive step for the operator to decide on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

tests/fixtures/discord_attachment_sidecar.json

@@ -0,0 +1,28 @@
+{
+  "author": "example-artist",
+  "author_files": [],
+  "author_id": "100000000000000001",
+  "category": "discord",
+  "channel": "nsfw-drops",
+  "channel_id": "200000000000000002",
+  "channel_topic": "",
+  "channel_type": 0,
+  "date": "2024-07-16 16:43:23",
+  "extension": "png",
+  "filename": "image",
+  "files": [],
+  "is_thread": false,
+  "message": "",
+  "message_id": "300000000000000003",
+  "num": 1,
+  "owner_id": "100000000000000001",
+  "parent": "",
+  "parent_id": "",
+  "parent_type": 0,
+  "server": "Example Server",
+  "server_files": [],
+  "server_id": "400000000000000004",
+  "subcategory": "channel",
+  "type": "attachment",
+  "url": "https://cdn.discordapp.com/attachments/200000000000000002/500000000000000005/image.png"
+}