perf: retry the bsky HLS request, not the whole fetch

A failed bsky video remux was reported as `FetchError::Transient`, so the
fetch loop retried the *whole* adapter — master playlist, variant playlist
and up to 500 segments again (256 MiB of cap each time), for a failure that
happened near the end of the work the retry was about to redo. The message
had to stay honest (returning `Ok` with no media reads as "this post has no
media"), so it needed a class of its own.

`FetchError::MediaPrep` is that class: post fetched, media could not be
prepared locally, not retryable (the per-site `is_retryable` whitelist
excludes it by construction), with its own user-facing text — the generic
"failed to fetch" would have hidden that the download or encode was what
broke. The retry itself is not lost, it moved: `fetch_hls` retries the
request that actually failed, once, for the classes a retry can change
(transport, 429/5xx).

Checked the sibling sites before widening the change: pixiv already degrades
to no media when the ugoira encode fails (`Ok(None)`), and its frame-zip
download failure is the last step so a re-fetch re-does only that; twitter's
auth leg replays two cheap metadata GETs and a GraphQL 5xx *is* worth
retrying. Neither needed the new class.

Verified with a throwaway proxy harness: a 503 answered twice-in-a-row path
costs 2 requests and succeeds, a 404 costs exactly 1 and fails (no pointless
retry). `site::bsky::interface::tests::media_prep_failure_is_not_retried`
pins the classification.

`cargo fmt --check`, `cargo clippy --workspace --all-targets --locked -- -D
warnings` and `cargo test --workspace --locked` clean.

AGENTS.md

@@ -66,7 +66,7 @@ Docker: `docker build -t tgxmb .` then `docker run --rm -d --name tgxmb --env-fi
 
 ## Code Conventions & Common Patterns
 
-- **Errors via `thiserror` derive** (no anyhow): the public, stringified errors — `FetchError` (`Http`/`Json`/`Pixiv`/`Site`/`NotFound`/`Blocked`/`Disabled`/`Sensitive`/`TooLarge`/`Transient`/`Io`) and `PixivError` — derive `thiserror::Error` with `#[from]` conversions; `Display`/`source()` come from the derive. The internal control-flow enums — `QueueError` (`Retryable { delay_seconds, payload }` / `Permanent`), `SendError` (Retryable/Permanent), `Classification`, `FallbackError` — carry no `Display` and are handled by direct variant matching. New errors should follow the same split: stringified/public errors derive `thiserror`, internal flow enums stay plain.
+- **Errors via `thiserror` derive** (no anyhow): the public, stringified errors — `FetchError` (`Http`/`Json`/`Pixiv`/`Site`/`NotFound`/`Blocked`/`Disabled`/`Sensitive`/`TooLarge`/`MediaPrep`/`Transient`/`Io`) and `PixivError` — derive `thiserror::Error` with `#[from]` conversions; `Display`/`source()` come from the derive. The internal control-flow enums — `QueueError` (`Retryable { delay_seconds, payload }` / `Permanent`), `SendError` (Retryable/Permanent), `Classification`, `FallbackError` — carry no `Display` and are handled by direct variant matching. New errors should follow the same split: stringified/public errors derive `thiserror`, internal flow enums stay plain.
 - **Global state via `std::sync::LazyLock` statics**, not DI: `CONFIG`, `CHAT_STORE`, `TASK_QUEUE` in `handlers/statics.rs`; shared reqwest `CLIENT` in `x-media/src/site/mod.rs`. `Bot` is passed/cloned into handlers; queue workers share the process-wide `send::BOT` (`LazyLock<Bot>`, force-initialized in `main` so a missing token fails at startup).
 - **Async**: tokio multi-thread runtime (`#[tokio::main]` default). All rusqlite I/O inside `tokio::task::spawn_blocking`. Long loops use `tokio::select!` with `tokio::sync::{watch, Notify}` stop/wake channels. No streams.
 - **Blocking sync primitives**: `parking_lot::Mutex` for hot caches, `tokio::sync::Mutex` for async-shared state (pixiv token cache), `AtomicBool` for feature gates.