feat: maintained next branch with a backup provider for outages

Is your feature request related to a problem? Please describe.
One provider is a single point of failure. gogoanime, animixplay, allanime, anidb.app: every switch left the tool broken for everyone until a patch shipped.

Describe the solution you'd like
As agreed with @port19x below: no failover inside the script. Instead a permanently maintained next branch, which is master with a second provider already wired up. On an outage a maintainer flips it live, early adopters run ani-cli -U next, and days become hours.

Scope:

• Pick a backup provider (none chosen yet, suggestions welcome)
• next = master with only the hianime_* scraper block swapped, kept in sync with master so the diff stays small
• Add next to the push branches in .github/workflows/ani-cli.yml so shellcheck/shfmt run there too
• Open the provider PR against next (branch exists, created at current master)

Deferred for now: an auto-merge workflow from master into next, and README docs for -U next.

Describe alternatives you've considered
Automatic in-script failover, the original proposal, rejected as too much change for one fallback. Patching on each outage, the current approach.

Additional context
hacking.md rejects multi-source support; this stays one provider per branch. I'll implement it, first results planned for the weekend of 19 September.

Start by reviewing the existing next branch, the hianime_* scraper block, and .github/workflows/ani-cli.yml. Done means next stays synchronized with master while retaining only the provider swap, and shellcheck/shfmt run for pushes to next; consult hacking.md for the project's single-provider constraint.