[codex] Add category-specific funding goal caps

[MyouzzZ]

Summary

• add per-category funding goal cap storage keyed by campaign category
• expose admin set/remove helpers plus a getter for the effective category cap
• enforce the stricter of the category cap and global max during create_campaign
• add focused tests for enforcement, category isolation, removal, validation, and non-admin rejection

Fixes #509.

Validation

• git diff --check
• static brace-balance sanity check on touched Rust files

I could not run cargo fmt / cargo test locally because this Windows environment does not have cargo or rustc installed.

[davidmaronio]

the category-specific funding-goal caps look reasonable but the pr is dirty, it conflicts with main (it touches admin.rs/storage.rs/lib.rs/create.rs which have moved). please rebase and resolve, i'll re-review the merged result (especially admin.rs/storage.rs for any silent clobber) and re-run ci before merge.