Close gaps in grouping, regression detection, alerts, ownership, environments, and session

[MdSadiqMd]

Closes #30

Summary

Add environment, session context, and regression detection to close remaining gaps in the Mesher backend ingestion and alerting pipeline.

What changed:

• Added environment and session_id columns to events table for deployment filtering and session correlation
• Added last_resolved_at timestamp to issues table for regression tracking
• Implemented regression detection that fires alerts when resolved issues receive new events
• Added session events API endpoint for chronological session replay
• Added environment filter to issue search
• Added assignment broadcasting following existing success pattern
• Added missing Discarded status to IssueStatus enum

Why:
Operators need trustworthy grouping, regression detection, environment filtering, and session context without relying on JSONB decomposition or overclaimed capabilities. This makes alerts, ownership, environments, and session context operator-usable.

Verification

# Run migration
cargo run -q -p meshc -- migrate mesher up

# Start Mesher backend
./mesher/mesher

# Test environment filtering (in another terminal)
curl "http://127.0.0.1:8080/api/v1/projects/default/issues?environment=production"

# Test session events endpoint
curl "http://127.0.0.1:8080/api/v1/projects/default/sessions/test-session-123/events"

# Test regression detection
# 1. Create and resolve an issue
curl -X POST http://127.0.0.1:8080/api/v1/events \
  -H "x-sentry-auth: mshr_devdefaultapikey000000000000000000000000000" \
  -H "Content-Type: application/json" \
  -d '{"message":"Test regression","level":"error","fingerprint":"test-regression-fp"}'

curl -X POST "http://127.0.0.1:8080/api/v1/issues/{issue_id}/resolve"

# 2. Send new event with same fingerprint
curl -X POST http://127.0.0.1:8080/api/v1/events \
  -H "x-sentry-auth: mshr_devdefaultapikey000000000000000000000000000" \
  -H "Content-Type: application/json" \
  -d '{"message":"Test regression again","level":"error","fingerprint":"test-regression-fp"}'

# Verify regression alert fired and issue status is unresolved

# Test migration rollback
cargo run -q -p meshc -- migrate mesher down
cargo run -q -p meshc -- migrate mesher up