feat: granular Firehol level control (ENABLE_FIREHOL_LEVEL1/2/3)

[wolffcatskyy]

Summary

Closes #83. Previously ENABLE_FIREHOL was an all-or-nothing switch for all three Firehol levels. This adds per-level control:

• ENABLE_FIREHOL_LEVEL1, ENABLE_FIREHOL_LEVEL2, ENABLE_FIREHOL_LEVEL3
• Each level flag falls back to the master ENABLE_FIREHOL when unset, so existing configs behave identically.
• When set, a per-level flag overrides the master for that level only.

Example: enable only level1

ENABLE_FIREHOL=false
ENABLE_FIREHOL_LEVEL1=true

Implementation

• Each Firehol BlocklistSource now uses its own enabled_key (enable_firehol_level1/2/3).
• Config.from_env() computes each level from ENABLE_FIREHOL_LEVELn if present, else from the master ENABLE_FIREHOL.
• .env.example and the --help text updated; VALID_ENABLE_VARS picks up the new keys automatically.

Verification

• Added 3 unit tests covering default-to-master, per-level override, and master-off behavior.
• Full suite: 228 passed, 6 skipped.
• End-to-end check confirms only the enabled level(s) are passed to the importer gate.

Backward compatible — no behavior change for users who only set ENABLE_FIREHOL.