Add spawner boost and crop growth boost reward types (#61)

[tastybento]

Summary

• Adds SpawnerReward / SpawnerRewardDB: listens to SpawnerSpawnEvent and spawns extra entities beside the triggered spawner, scaled by a per-tier formula
• Adds CropGrowthReward / CropGrowthRewardDB: listens to BlockGrowEvent on agricultural crops and applies extra applyBoneMeal() calls, scaled by a per-tier formula
• Both use a fractional-bonus approach (0.5 → 50% chance of one extra per trigger; 1.5 → always 1 extra plus 50% chance of a second)
• Compatible with WildStacker, RoseStacker, and any other stacker plugin — extra entities are spawned as raw entities which the stacker then handles normally
• Adds a public getUpgradeData() getter on DatabaseUpgrade (needed by listeners to resolve tiers)
• Registers both reward types and both listeners in UpgradesAddon.onEnable()
• Seeds 2 new example upgrades on first install (Spawner Boost @ $2000, Crop Growth Boost @ $500), bringing the default total to 8
• Adds upgrades.rewards.spawner.* and upgrades.rewards.cropgrowth.* locale keys to en-US.yml

Test plan

• mvn test — all 102 existing tests pass
• Start server with BSkyBlock; confirm 8 example upgrades are seeded
• Admin creates a Spawner Boost upgrade with formula 0.5, price $1000
• Player purchases it; no console errors
• Stand near a spawner — extra entities appear next to it on spawner ticks
• Admin creates a Crop Growth upgrade with formula 0.5, price $500
• Player purchases it; plant wheat on island — crop advances faster than a non-upgraded island
• Test with WildStacker enabled — extra entities are stacked normally
• Verify admin panel formula input validates and shows green/red status correctly

Closes #61

🤖 Generated with Claude Code