fix: prevent storing entries with Unix epoch timestamp (fixes #97)

[digizeph]

Summary

Fixes #97 by preventing erroneous broker entries with Unix epoch timestamp (1970-01-01) from being stored and returned in search results.

Changes

Bug Fixes

• Added MIN_VALID_TIMESTAMP constant (1998-01-01) in crawler/common.rs

  • The earliest collector (rrc00) started on 1999-10-01, so 1998 provides a safe buffer
  • Crawlers now filter out entries with timestamps before 1998-01-01 during parsing

• Added default 30-day time window for queries without timestamp filters

  • Prevents slow full table scans (12.9s → 1.0s, 13x faster)
  • Returns recent data by default instead of starting from 1970
  • Avoids returning phantom 1970 entries

Testing

• Added unit test test_min_valid_timestamp to verify the threshold
• All 60 unit tests pass
• All 62 doc tests pass
• Clippy checks clean

Production Impact

• Production database already cleaned: 3 phantom entries with timestamp=0 deleted from deployment (rrc00, rrc01, route-views2)
• The code changes prevent new erroneous entries from being inserted

Performance Improvement

Query Type	Response Time	Speedup
No timestamp filter	~12.9 seconds	Baseline
30-day window	~1.0 second	13x faster