feat(import): import indexer cart RSS feeds

[javi11]

Adds indexer cart RSS imports in Configuration → Import. Configure one feed URL and a polling interval (300 seconds by default); clear the URL to disable it. Settings apply without restarting.

The worker downloads NZBs through the configured proxy and User-Agent, validates them, and submits them to the normal import queue. Database receipts prevent repeat imports after restarts or queue/history cleanup. Failed downloads retry independently; interrupted enqueues recover without acknowledging missing content. Feed URLs are masked in API responses and excluded from HTTP errors.

Includes SQLite/PostgreSQL migration 042, configuration documentation, and integration tests. Real PostgreSQL testing also exposed ambiguous column references in queue insertion; those references are qualified so enqueueing works on PostgreSQL. The existing migration rollback test now explicitly targets migration 40.

Validation:

• Full Go suite with race detection: go test -race -p 4 ./... passed.
• Independent affected-package race tests with -count=1 passed.
• Real disposable PostgreSQL suite (Postgres|RSS) passed, including migrations and normal enqueue operations.
• go vet ./..., frontend production build, targeted Biome checks, and documentation production build passed.
• RSS worker statement coverage: 86.4%. Tests cover polling, restart deduplication, failed downloads, malformed payloads, proxy/User-Agent, cancellation, settings masking, redirected relative links, and interrupted enqueues.
• Independent review completed; both reported edge cases were corrected and retested.

The pinned golangci-lint cannot read Go 1.27 export data. The same tooling error reproduces on unchanged main; no toolchain/dependency changes are included.

Supports one RSS 2.0 feed and XML NZBs. Atom and compressed attachments are unsupported. Changing the feed URL starts a new deduplication namespace.

Closes #76