chore: Fix/cleanup tests, docs, specs

[Ziinc]

Around 50% of all tests are failing, this issue tracks the progress of getting the test suite back on track to 100%. Some tests are also out-of-date, redundant, overly-coupled, or missing, so this cleanup will serve to improve test coverage for current code.

Docs + specs are also missing for many core functions, so since we're going context-by-context and controller-by-controller, we can take this opportunity to add them in. Adding in specs would also bring us closer to enabling the dialyzer checks on CI.

The number of contexts are also getting a little out of hand, should try to consolidate where possible.

• logflare
  • admin chore: refactor admin context, cluster_live #1115
  • auth, chore: add docs & specs to functions, remove unused code #1126
  • billing_accounts chore: Billing context, rename BillingAccounts namespace #1118
  • cluster
  • contact chore: refactor Logflare.Contact context #1117
  • ecto
  • endpoint
  • google
  • logs
  • oauth_access_grants
  • oauth_access_tokens
  • oauth_applications
  • payment_methods chore: move payment methods context into billing context #1122
  • plans chore: consolidate plans context into billing #1124
  • pubsub_rates
  • queries chore: consolidate BillingCounts under Billing #1125
  • saved_searches
  • source
  • source_schemas
  • sources
  • system_metrics
  • team_users
  • teams chore: add docs/specs/tests for Teams context, renamed get_team_by! to get_team_by #1121
  • telemetry
  • tracker
  • users
  • vercel
  • account_email
  • billing_counts chore: consolidate BillingCounts under Billing #1125
  • cache_buster
  • context_cache
  • generators
  • mailer
  • source_schemas
  • sql
  • system_metric
  • user
• logflare_web`