feat: reporting suite — inventory, spend, usage, low-stock (PDF + CSV)

[crzykidd]

Major feature from PRD §10.4. Generate and export reports across ammo, spend, and usage.

Scope

• Inventory snapshot — all visible ammo with current quantity, value, and location.
• Spend report — total cost by caliber, by dealer, and by time period.
• Usage report — rounds expended over time, by caliber, by user, and by firearm.
• Low-stock report — all calibers below their configured threshold.
• Export every report to PDF and CSV.
• Optional scheduled report delivery via email (requires SMTP configured in config.yaml).

Notes

• The insurance-valuation report tracked in Feat: provide insurance readout report #17 is one report within this suite — keep them linked; Feat: provide insurance readout report #17 can be closed into this or kept as the specific sub-task.
• Respects the existing ownership/visibility model (Admin sees all; Members see shared + own; Read-Only sees shared).

Source: PRD §10.4 (originally tagged v2.0).