[config] restore backfill enable flag

[90-008]

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Motivation

• Restore the previous configuration guard that allowed deployments to disable the backfill worker so queued repository backfills (which resolve DID docs and fetch CARs from PDS endpoints) cannot be forced on by discovered accounts and cause outbound fetch/parse DoS exposure.

Description

• Reintroduce enable_backfill: bool on Config and include it in Display output (src/config.rs).
• Parse HYDRANT_ENABLE_BACKFILL in the environment loader and carry the value into Config::from_env() (src/config/env.rs).
• Initialize the runtime backfill_enabled watch channel from config.enable_backfill instead of unconditionally enabling it (src/state.rs).
• Keep default behaviour unchanged by defaulting enable_backfill to true in Config::default() so existing deployments are unaffected unless they explicitly opt out.

Testing

• Ran cargo check --locked with a workaround (temporarily moved aside the repository .cargo/config.toml that injects a custom linker flag) and the build finished successfully; this verifies the changes compile in a standard toolchain environment.
• Observed an initial cargo check --locked failure due to the local .cargo/config.toml passing an unsupported --ld-path=wild linker argument in this environment, which is unrelated to the changes and was worked around as described above.
• git diff --check passed and changes were committed as [config] restore backfill enable flag.

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Codex Task