Rename terminology: Source/Sink → Client/Destination

[brandonjackson]

Summary
Standardise language to be more user-friendly

Channels route requests from a Client → Destination

Why

• “Source/Sink” is confusing...is the "source" the user OR the data source?
• Source/Sink is from streaming paradigm, we are request/response
• Reinforces: Channels are dumb (routing only), Jobs handle logic

Changes

• Source → Client
• Sink / Upstream → Destination
• Apply across UI, docs, and copy

Acceptance

• No “Source/Sink” left in user-facing surfaces
• Channels consistently described as routing requests only