feat: cost analytics — price-per-round trends, spend by dealer

[crzykidd]

Major feature from PRD §10.5. Analytics over purchase cost data already captured per box.

Scope

• Price-per-round over time per caliber — surface whether prices are rising or falling.
• Total spend by dealer / source.
• Average cost per round by caliber across all purchases.
• Spend trend charts.

Notes

• Builds on the cost_per_round and dealer data already on every ammo box.
• Related but distinct from feat: AmmoSeek price integration #4 (AmmoSeek price integration) — that pulls external market prices; this analyzes your own purchase history. They could complement each other later.

Source: PRD §10.5 (originally tagged v2.0).