Accelerate source blob backups with resumable transfers

[paullizer]

Summary

• replace serial whole-blob buffering with bounded concurrent Azure SDK block transfers
• add durable per-file target verification, lease-generation fencing, Retry-After-aware adaptation, failure isolation, and restart-safe reuse
• preserve optional encryption with authenticated, source-bound, ordered Fernet chunk frames
• add dedicated blob concurrency/chunk/retry settings, progress telemetry, admin UI coverage, and version 0.250.102
• add a credential-safe benchmark harness and document the AzCopy, server-side copy, and SDK transport decision

Benchmark evidence

The harness enforces a 50 GiB source-set threshold by default and records verified count, bytes, elapsed time, and throughput for each compatible candidate. Live results remain environment-specific and must be generated in an approved operator-provided Azure environment; the repository does not fabricate them.

Validation

• 39 passed, 1 skipped across focused blob transfer, backup durability, Cosmos/Search backup, security, and admin UI tests
• Python compilation, JavaScript syntax, benchmark CLI, and diff checks passed

Fixes #1095