Add tax exemption attribute for firms

Code context — this is largely already implemented. A per-firm balance-tax.exempt flag exists: FirmPropertyService.setBoolean, settable via /business tax exempt <firm> <bool> (TaxCommands, perm business.tax.exempt), and FirmBalanceTaxListener.buildFirmCollections honors it (exempt firm → zero collections).

I traced the weekly tax cycle end-to-end and it works: Treasury schedules + fires TaxCycleEvent(WEEKLY) (enabled in config, Treasury.onEnable), business registers the listener (Business.java:75), and non-exempt firms are charged via collectBatch → ledger to DCGovernment.

Remaining gap vs the title ("not visible under the treasury… tag rather than configure"): exemption status isn't surfaced in the explorer/treasury views. Recommend narrowing this to "surface firm exemption status in the explorer," or verify/close. (Tiny separate follow-up spotted: the listener never calls taxApi.registerCycleParticipant("Business", WEEKLY), so Business doesn't appear in /tax status — observability only, collection is unaffected.)

Done — exemption is now surfaced in the explorer (economy-explorer @ ccaa70b, develop).

Findings on the existing mechanism (verified by code read): exemption is already a tagged firm attribute (firm_properties, key balance-tax.exempt), already honored by the weekly cycle (FirmBalanceTaxListener.buildFirmCollections → exempt firm returns no collections), and already shown in-game via /business tax info ("Exempt"/"Not Exempt"). The only gap was web visibility ("not visible under the treasury").

Change (explorer):

• findFirmByDisplayName (lib/sql/firm.ts) now reads the flag via a correlated subquery on firm_properties, filtering type='BOOLEAN' and deleted_at IS NULL and parsing 'true' case-insensitively — exactly mirroring FirmPropertyServiceImpl.getBoolean.
• Firm detail page shows a green "Tax exempt" badge next to the firm name (alongside the Archived badge).
• Integration schema/seed gain firm_properties, an actively-exempt firm, and a soft-deleted flag on Acme to prove the deleted_at filter. getFirmStats expectations updated for the added firm.

Validated: typecheck + lint + 55 unit tests green. Integration test runs in CI (needs the MariaDB service container; couldn't run locally — no DB creds in this environment).

Deferred follow-up (not done — out of the chosen scope): the listener still never calls taxApi.registerCycleParticipant("Business", WEEKLY), so Business doesn't appear in Treasury's /tax status. Pure observability; worth a small separate ticket if wanted.

State corrected Pending Release → Done: the firm tax-exemption command/attribute (TaxCommands, FirmBalanceTaxListener, BalanceTaxConfiguration) is shipped — fix commit d260686 is on business main (released), not just develop.