feat(channels/discord): outbound attachments with SSRF-guarded Multipart batching

Squashed reroll of topic/discord-outbound-attachments onto v0.6.9 (acf2587).
Replaces the 16-commit history that landed via local-main merge 2c32102.

Reroll context:
  - mime/size on ChannelContent::File and file:// in download_image_to_blocks
    already landed upstream via #1143; dropped from this branch.
  - source_url-on-ContentBlock::Image is provided by feat/runtime-image-cache
    (PR #1151); the three compactor source_url tests live there.
  - Net contribution here is the outbound Multipart feature plus its deps.

Feature scope:
  - Outbound for ChannelContent::File and Image URL arms (per-block resolver
    dispatch in mixed Multipart payloads).
  - N-attachment batching with greedy-pack chunker and aggregate per-chunk
    byte cap; parallel attachment fetch via try_join_all.
  - SSRF guard + redirect revalidation + log scrubbing for URL fetch
    (Fetcher trait abstraction).
  - Body-aware 429 retry on the multi-file path; structured partial-send
    WARN with grep-friendly event.
  - Transparent decompression disabled on image download (bridge.rs).

Verification:
  - cargo check --workspace: clean
  - cargo test -p openfang-channels -p openfang-runtime: 1002/1002

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: benhoverter

Cargo.lock

@@ -29,6 +29,7 @@ sha1 = { workspace = true }
 aes = "0.8"
 cbc = "0.1"
 base64 = { workspace = true }
+bytes = { workspace = true }
 hex = { workspace = true }
 html-escape = { workspace = true }
 regex-lite = "0.1"