fix(syntaxis): correct download dispatch (slot leaks, retry wakeup, protocol guard)

[forkwright]

Closes #393, #394, #424, #425, #426, #427, #428.

Changes

• start_download failure permanently leaks a concurrency slot, active entry, and DB row #393 — slot/row leak. A start_download failure leaked the concurrency slot, active entry, and left the row non-terminal. The dispatch logic is now one shared helper that releases the slot, removes the active entry, marks the row failed, and pulls a replacement.
• Retry re-enqueue after backoff never wakes the dispatcher #394 — dead retry wakeup. The retry re-enqueue never woke the dispatcher; the spawned retry task now calls the dispatch helper after backoff.
• Download dispatch hardcodes Torrent protocol, breaking all NZB/Usenet downloads #424 — protocol mis-route. Dispatch hardcoded Torrent, silently mis-routing NZB/Usenet to the BitTorrent engine. It now matches on protocol and fails a Usenet item loudly (marked-failed, slot rolled back) instead. Full Usenet support is a separate feature.
• TOCTOU between has_slot check and acquire in priority-4 enqueue allows active_total to exceed max_concurrent #425 — TOCTOU. The priority-4 has_slot/acquire/insert collapse into one critical section; losers demote to priority 3.
• Dropped broadcast DownloadCompleted events permanently leak download slots #426 — dropped-event slot leak. On broadcast Lagged (and a 60s periodic pass) active downloads are reconciled against the engine, so dropped DownloadCompleted events no longer permanently leak slots.
• Retry budget resets to zero on every dispatch — defeated across process restarts #427 — retry budget across restarts. retry_count is carried on QueueItem and restored from the DB.
• No regression test that a retried download is re-dispatched after backoff #428 — regression test that a retried download is re-dispatched after backoff.

Verification

kanon gate --full green (fmt, check, advisory-parity, cargo-deny, clippy workspace, nextest, kanon lint). 179 tests across syntaxis + archon.