Validate poll interval is positive to prevent ticker panic

[Copilot]

The daemon's poll interval configuration was not validated, allowing zero or negative durations that cause time.NewTicker to panic at runtime.

Changes

• Add validation after parsing RATELORD_POLL_INTERVAL environment variable
• Add validation after parsing --poll-interval command-line flag
• Both reject durations ≤ 0 with clear error messages
• Add test coverage for zero, negative, and invalid format cases

Example

# Previously would panic at runtime
RATELORD_POLL_INTERVAL=0s ratelord-d
# Now returns: "RATELORD_POLL_INTERVAL must be positive"

ratelord-d --poll-interval=-5s
# Now returns: "poll interval must be positive"

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