fix JWK endpoints returning empty keys

[olamilekan000]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

The JWKS endpoint (/auth/v1/.well-known/jwks.json) returns an empty keys array ({"keys": []}) for local Supabase instances because signing keys are not automatically generated during supabase init. Users must manually generate signing keys using supabase gen signingkeys and configure signing_keys_path in config.toml.
Issue: #4614

What is the new behavior?

• Automatically generates a default RS256 signing key during supabase init
• Saves the key to supabase/signing_keys.json (already in .gitignore)
• Skips generation if the signing key file already exists
• The JWKS endpoint now returns the public key, matching cloud behavior

Additional context

Add any other context or screenshots.

[olamilekan000]

@sweatybridge kindly review. Thanks

[coveralls]

Pull Request Test Coverage Report for Build 20494065924

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 28 of 34 (82.35%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 56.133%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/init/init.go	28	34	82.35%

Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 20484757308:	0.04%
Covered Lines:	6864
Relevant Lines:	12228

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💛 - Coveralls