feat(stremio): instant mount, X-Api-Key auth, NZB by URL, cached flag

[javi11]

Summary

Makes AltMount a first-class backend for AIOStreams (and any Stremio integration) by closing the latency gap between NZB submission and first playable frame.

Changes

Instant mount (A1 — internal/importer/service.go, internal/progress/progress_broadcaster.go)

• ProgressBroadcaster now has a NotifyStreamable(queueID, storagePath) method that broadcasts a "streamable" event the moment StoragePath is written to the DB — before post-processing (symlinks, STRM files, health scheduling) and before status=Completed.
• handleProcessingSuccess in the importer service calls this immediately after AddStoragePath.

Early stream return (A1 — nzb_stremio_handlers.go, stremio_addon_handlers.go)

• waitAndRespond (native /api/nzb/streams) and waitAndRedirectToStream (addon play path) both listen for "streamable" events and return/redirect immediately — no longer blocked on status=Completed.
• Upfront check also catches items already in processing state with a storage path set (covers the subscribe-after-event race).

X-Api-Key header auth (A5 — nzb_stremio_handlers.go)

• POST /api/nzb/streams now accepts the raw API key via X-Api-Key header in addition to the download_key form field (SHA256 hash). AIOStreams sends altmountApiKey in X-Api-Key — this makes it work without any extra config on the AIOStreams side.

NZB by URL (A5 — nzb_stremio_handlers.go)

• New nzb_url form field: callers pass an NZB download URL instead of uploading file bytes, removing an extra HTTP round-trip.

_cached flag (A5 — nzb_stremio_handlers.go)

• StremioStreamsResponse gains _cached: bool. Set to true when streams are served from an already-completed queue item. AIOStreams can use this to show a lightning/instant badge without probing WebDAV.

Test plan

• Single MKV NZB: submit via POST /api/nzb/streams with X-Api-Key header → confirm response arrives before status=Completed (streamable stage) and _cached=false
• Same NZB a second time → _cached=true, response is instant
• nzb_url field: pass an NZB download URL instead of file upload → same result
• RAR archive NZB: confirm streams returned as soon as RAR headers are parsed and metadata written
• Failed/dead release: confirm clean HTTP error response, no hang
• Addon play path (GET /stremio/:key/play): confirm 302 redirect fires at streamable stage, not at completed
• Existing Plex/WebDAV behaviour unchanged (post-processing still runs to completion async)
• go test ./internal/api/... ./internal/importer/... ./internal/progress/... passes