Storage layer

[nccapo]

Here's a summary of what's new

memstore.go

• MemStore struct with sync.RWMutex protecting five maps: events, groups, fingerprint index ("projectID:fingerprint" → groupID), crashes, projects, and a DSN
  key index
• Deep copy helpers for all domain types — callers can't mutate stored state
• Generic paginate[T] helper (limit=0 means no cap)
• ListGroups sorts by last_seen/first_seen/count descending; ListEvents and ListPodCrashes sort newest-first
• IncrementGroupCount and UpdateGroupStatus mutate in place (no re-save needed)

memstore_test.go — 39 tests across every method:

• Save + Get happy paths for all four entity types
• ErrNotFound for missing IDs and wrong-project isolation
• Copy semantics verified (mutating returned structs doesn't corrupt store)
• ListEvents: project filter, level filter, descending order, limit/offset
• ListGroups: status filter, all three sort fields, limit/offset
• ListPodCrashes: namespace filter, descending order, limit/offset
• IncrementGroupCount / UpdateGroupStatus not-found cases
• DeleteEventsOlderThan: partial delete, none match, empty store

Design decisions:

• ZSTD compression via options.ZSTD
• Reverse iterator (reverseIter helper) serves events/crashes newest-first directly from key ordering — no in-memory sort needed for single-prefix scans
• Groups/cross-namespace listing loads into memory and sorts (arbitrary sort fields can't be served from key layout)
• Secondary lookup indexes (ei:, ci:) enable O(1) point lookups by ID without knowing the timestamp at call time
• DeleteEventsOlderThan parses timestamps directly from keys (no JSON decode) via parseEventKey, then uses WriteBatch for bulk deletion; also cleans up lookup
  index entries
• startGC runs db.RunValueLogGC(0.5) every 5 minutes in a goroutine stopped cleanly by Close()

badger_test.go — Integration tests (74 tests total across both files)

• Mirrors every memstore scenario: save+get, not-found, wrong-project isolation, list filters, sort orders, pagination, increment, status update, delete old
  events
• TestBadger_DataSurvivesReopen — closes and reopens the DB to confirm ZSTD-compressed data persists
• TestBadger_Event_RoundTrip — verifies all field values survive JSON encode/decode
• TestParseEventKey — unit tests the key-parsing helper used by DeleteEventsOlderThan