docs(diagrams): added C4 diagrams

assets/plantuml/raw/c4-component-critical-path.puml

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+@startuml
+!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Component.puml
+
+LAYOUT_TOP_DOWN()
+
+title LOTTY Backend - C4 Component
+
+Container_Boundary(api, "Backend Container (Django + Django Ninja)") {
+  Component(decision_endpoint, "Decision Endpoint", "api/v1/decision/endpoints.py", "POST /api/v1/decide")
+  Component(decision_service, "Decision Service", "apps/decision/services.py", "Flag lookup, targeting DSL, participation limits, hash-based variant assignment")
+
+  Component(events_endpoint, "Events Endpoint", "api/v1/events/endpoints.py", "POST /api/v1/events")
+  Component(events_service, "Events Service", "apps/events/services.py", "Validation, dedup, exposure/conversion attribution, pending promotion")
+
+  Component(reports_endpoint, "Reports Endpoint", "api/v1/reports/endpoints.py", "GET /reports/{experiment_id}")
+  Component(reports_service, "Reports Service", "apps/reports/services.py", "Per-variant metric calculation (ratio, count, average, percentile)")
+
+  Component(guardrails_service, "Guardrails Service", "apps/guardrails/services.py", "Threshold checks, auto pause/rollback")
+}
+
+ContainerDb(db, "Relational DB", "PostgreSQL / SQLite", "Flags, experiments, variants, decisions, events, exposures")
+Container(cache, "Cache", "Valkey / LocMem", "Flag and active experiment cache")
+Container(worker, "Celery Beat", "Celery", "Periodic guardrail checks (60s)")
+
+Rel(decision_endpoint, decision_service, "Delegates")
+Rel(decision_service, cache, "Reads cached flag / active experiment")
+Rel(decision_service, db, "Reads variants, writes Decision")
+
+Rel(events_endpoint, events_service, "Delegates batch")
+Rel(events_service, db, "Writes Event / Exposure / PendingEvent")
+
+Rel(reports_endpoint, reports_service, "Builds report")
+Rel(reports_service, db, "Reads attributed events / exposures")
+
+Rel(guardrails_service, reports_service, "Uses metric calculations")
+Rel(guardrails_service, db, "Writes triggers, updates experiment status")
+Rel(worker, guardrails_service, "Periodic check_all")
+
+@enduml

assets/plantuml/raw/c4-container.puml

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+@startuml
+!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
+
+LAYOUT_LEFT_RIGHT()
+
+title LOTTY - C4 Container
+
+Person(product_client, "Product Client", "Web/Mobile app")
+Person(ops_user, "Experimenter/Approver/Admin/Viewer")
+
+System_Boundary(lotty_boundary, "LOTTY Backend") {
+  Container(api, "Backend API", "Django + Ninja", "REST endpoints: flags, experiments, decide, events, reports, guardrails, notifications, conflicts, metrics, learnings, reviews, users, auth")
+  ContainerDb(db, "PostgreSQL / SQLite", "Relational DB", "Experiments, variants, decisions, events, exposures, outcomes, guardrails, notifications, conflicts, logs")
+  Container(cache, "Valkey / LocMem Cache", "Cache + Celery broker", "Flag/experiment cache, Celery task broker")
+  Container(worker, "Celery Worker / Beat", "Background worker", "Periodic: guardrails check, notifications flush, pending events cleanup")
+}
+
+System_Ext(notifications, "Notification Channels", "Telegram / SMTP")
+
+Rel(product_client, api, "Gets flag decisions, sends events", "HTTP/JSON")
+Rel(ops_user, api, "Admin/experiment/review calls", "HTTP/JSON")
+Rel(api, db, "Read/write application state", "SQL")
+Rel(api, cache, "Read/write cache", "Redis protocol")
+Rel(worker, db, "Read/write application state", "SQL")
+Rel(worker, cache, "Receives tasks, reads cached state", "Redis protocol")
+Rel(worker, notifications, "Delivers queued notifications", "HTTP / SMTP")
+
+@enduml

assets/plantuml/raw/c4-context.puml

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+@startuml
+!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml
+
+LAYOUT_LEFT_RIGHT()
+
+title LOTTY - C4 Context
+
+Person(product_client, "Product Client", "Web/Mobile app")
+Person(analyst, "Experimenter", "Creates experiments and analyzes outcomes")
+Person(approver, "Approver", "Reviews and approves experiments")
+Person(admin, "Admin", "Manages users, roles, review policies")
+Person(viewer, "Viewer", "Can only view")
+
+System_Boundary(lotty_boundary, "LOTTY Backend") {
+  System(lotty, "LOTTY Backend", "Django + Ninja API", "Feature flags, experiments, decisions, events, reports, guardrails, notifications, conflicts")
+}
+
+System_Ext(notification_channels, "Notification Channels", "Telegram / SMTP", "Receives operational notifications")
+System_Ext(observability, "Observability Stack", "VictoriaMetrics / Grafana / Loki / Tempo", "Metrics, logs, traces")
+
+Rel(product_client, lotty, "Gets flag decisions, sends events", "HTTP/JSON")
+Rel(analyst, lotty, "Configures experiments, views reports", "HTTP/JSON")
+Rel(approver, lotty, "Approves/rejects experiments", "HTTP/JSON")
+Rel(admin, lotty, "Manages users and review settings", "HTTP/JSON")
+Rel(viewer, lotty, "Views objects on platform", "HTTP/JSON")
+Rel(lotty, notification_channels, "Sends experiment/guardrail notifications", "HTTP / SMTP via Celery worker")
+Rel(lotty, observability, "Exports metrics, logs, traces", "OTLP")
+
+@enduml