Upgrade jjwt 0.11.2 -> 0.12.6

[devin-ai-integration]

Summary

Bumps jjwt-api/jjwt-impl/jjwt-jackson to 0.12.6 and migrates DefaultJwtService to the 0.12 API (the 0.11 parserBuilder()/parseClaimsJws() path is gone).

-signatureAlgorithm = SignatureAlgorithm.HS512;
-signingKey = new SecretKeySpec(secret.getBytes(), signatureAlgorithm.getJcaName());
+signingKey = Keys.hmacShaKeyFor(secret.getBytes());

-Jwts.builder().setSubject(id).setExpiration(exp).signWith(signingKey)
+Jwts.builder().subject(id).expiration(exp).signWith(signingKey)

-Jwts.parserBuilder().setSigningKey(signingKey).build().parseClaimsJws(token).getBody()
+Jwts.parser().verifyWith(signingKey).build().parseSignedClaims(token).getPayload()

Note on the key: the old code hard-coded the JCA name HmacSHA512 on the SecretKeySpec, but signWith(key) in 0.11 still selected the algorithm purely from key length. In 0.12 a key whose JCA name says HS512 while being shorter than 512 bits no longer resolves to any algorithm, which would break the 480-bit secret used in DefaultJwtServiceTest. Keys.hmacShaKeyFor picks the JCA name from the key length, preserving the previous behavior (HS512 for the 688-bit configured jwt.secret).

Verification: ./gradlew clean test spotlessCheck — 68 tests, 0 failures, Spotless clean. jacocoTestCoverageVerification still fails at 0.33 line coverage, which is pre-existing on main (the repo's test command excludes that task).

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/21599492e24a4fff86cd548540f5e681
Requested by: @mbatchelor81

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.