Add treasury reserve capacity status

[ramimbo]

Summary

Adds read-only treasury reserve capacity visibility for maintainers and agents:

• public GET /api/v1/treasury/status
• /admin treasury reserve capacity panel with pending create-bounty proposals
• docs for checking capacity before opening fresh rounds
• regression coverage for status output and admin rendering

This is a maintainer PR. I am not claiming or requesting a bounty for it.

Why

Fresh bounty rounds are now proposal-gated and reserve-capped. Maintainers need to see the rolling 24h reserve cap, executed reserves, pending create-bounty reserve, remaining create capacity, and next release time before deciding whether to open or wait on new rounds.

Validation

• ./.venv/bin/python -m pytest (483 passed)
• ./.venv/bin/python -m ruff format --check .
• ./.venv/bin/python -m ruff check .
• ./.venv/bin/python -m mypy app
• ./.venv/bin/python scripts/docs_smoke.py

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