fix: retry bilibili range proxy fetches

[Priveetee]

Summary

• retry BiliBili range upstream fetches before responding to dashjs
• add BiliBili range headers for Akamai requests
• cover retry behavior with a focused test

Verification

• docker JDK 21 ./gradlew test --tests dev.typetype.server.BilibiliRangeProxyTest --no-daemon
• docker JDK 21 ./gradlew clean check --no-daemon
• docker JDK 21 ./gradlew shadowJar --no-daemon
• runtime curls on BV1GpibBoEPH?p=1 deep ranges via temporary corrected server