Name the two server secrets after what they protect

[guohai]

Why

The old names didn't say what each secret is for. CONNECTION_ENCRYPTION_KEY is used only for Agora Console connections, and SPACE_CHANNEL_SECRET only for spaces' Agora Signaling channels.

What changes

Old name	New name	Used for
CONNECTION_ENCRYPTION_KEY	AGORA_CONSOLE_ENCRYPTION_KEY	Encrypting stored Agora Console tokens
SPACE_CHANNEL_SECRET	SPACES_SIGNALING_SECRET	Deriving unlisted and private spaces' Signaling channel names and keys

• The old names are still read as fallbacks, and the new name wins when both are set. A deployment that already uses the old names keeps working, keeps its encrypted connections, and keeps the same private channels.
• .env.example, the README and docs/platform.md use the new names.

Verification

• npm test: 296 pass, 0 fail, 1 skipped (PostgreSQL). New tests check each new name, the fallback, and that the new name wins.
• Redeployed locally under the new name: the Stoa loads and Signaling tokens are issued.

Generated with SMT smt@agora.build