fix: handle negative pg_class.reltuples in PostgreSQL Count functions

[legenduck]

Problem

makeslice: cap out of range panic when using PostgreSQL as data store on fresh/empty database.

Occurs on first startup before ANALYZE runs, because pg_class.reltuples returns -1 (float4) for unanalyzed tables.

Root Cause

CountUsers and CountItems in storage/data/sql.go scan reltuples directly into int64, while CountFeedback correctly uses float64. The -1 passes through to make([]..., -1) → panic.

Fix

• Scan reltuples as float64 first, then convert (consistent with CountFeedback)
• Clamp negative values to 0 for all three Count functions

Reproduction

1. Configure Gorse with PostgreSQL + Redis Stack
2. Start with empty database (no prior ANALYZE)
3. Master panics at "load dataset" step:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.71%. Comparing base (4ed9467) to head (bde0e94).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1186      +/-   ##
==========================================
- Coverage   73.08%   72.71%   -0.37%     
==========================================
  Files          88       88              
  Lines       16254    16258       +4     
==========================================
- Hits        11879    11822      -57     
- Misses       3202     3235      +33     
- Partials     1173     1201      +28     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.