hls: retire legacy basename-keyed HLS files on startup

Adds `video::legacy_migration::retire_legacy_hls_output`, called once
from `main` right after the diesel migrations run and before the
actor pipeline starts. Walks `$VIDEO_PATH` at depth 1, deletes every
`.m3u8` / `.m3u8.tmp` / `.m3u8.unsupported` / `.ts` file at root, and
logs a single info line with per-class counts. Skips directories
(the new layout's `<shard>/<hash>/` lives there) and unknown
extensions, so an operator's stashed README or `.tmp` from a
different tool is safe.

Why this needs its own one-shot pass rather than letting the rewritten
`cleanup_orphaned_playlists` handle it: the cleanup walk deliberately
only looks at `<shard>/<hash>/` dirs (so it can't accidentally `rm`
operator-stashed content), so without this migration the legacy files
would sit at root forever, never served, never refreshed. Operator
complaint count from the previous IMG_NNNN.MOV collision: ~10
duplicate-basename hits on one library alone; total .m3u8 count was
699 vs a much larger video count — i.e. the loser of every collision
was a permanent orphan. This pass collects all of them, then the
running watcher writes hash-keyed playlists going forward.

Idempotent — a second boot finds nothing and reports zero deletions,
so the call site can stay in `main` across releases until the module
is removed in a later cleanup commit. Tests cover the happy path
(legacy artifacts gone, hash dir untouched, unrelated files left
alone), idempotency, and the missing-directory case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -72,6 +72,16 @@ fn main() -> std::io::Result<()> {
 
     run_migrations(&mut connect()).expect("Failed to run migrations");
 
+    // One-shot retirement of the pre-content-hash HLS layout. Idempotent
+    // — a second boot finds nothing and reports zero deletions, so it's
+    // safe to leave wired in until the module is removed in a later
+    // release. Runs before the actor pipeline starts so we never race a
+    // PlaylistGenerator write against this rm.
+    {
+        let video_path = env::var("VIDEO_PATH").expect("VIDEO_PATH was not set in the env");
+        video::legacy_migration::retire_legacy_hls_output(std::path::Path::new(&video_path));
+    }
+
     let system = actix::System::new();
     system.block_on(async {
         // Just use basic logger when running a non-release build