add cooldown parameter to filtered-camera

[bhaney]

Core logic — image_buffer/image_buffer.go:

• Added cooldownSecs and cooldownTill fields to ImageBuffer struct
• Updated NewImageBuffer constructor with cooldownSecs parameter
• MarkShouldSend now computes cooldownTill = captureTill + cooldownSecs
• New IsInCooldown(now) method — returns true when past capture window but within cooldown period
• New SetCooldownTill test helper
• When cooldownSecs == 0, IsInCooldown naturally returns false (no cooldown)

Filtered camera — cam.go:

• Added CooldownSecs int with json:"cooldown_s" to Config
• Validation: cooldown_s cannot be negative
• Passed to NewImageBuffer in constructor
• Added cooldown check in images() between IsWithinCaptureWindow and filter checks — during cooldown, drains remaining buffered images but suppresses new triggers

Conditional camera — conditional_camera/conditional_cam.go:

• Same config, validation, constructor, and images() cooldown check

Tests:

• 3 new unit tests in image_buffer_test.go: TestCooldownBlocksRetrigger, TestCooldownZeroHasNoEffect, TestCooldownExtendsWithRetrigger
• 3 new integration tests in cam_test.go: TestCooldownValidation, TestCooldownSuppressesNewTrigger, TestCooldownAllowsTriggerAfterExpiry
• All existing NewImageBuffer calls updated with 0 for backward compatibility

[kharijarrett]

LGTM! Glad we'll still clear out the buffer even in cooldown