feat(server): add PCDN scheduling & dispatch pipeline

[hequan2017]

Motivation

• Provide a PCDN scheduling and delivery pipeline to select edge nodes for content delivery based on region/ISP/real-time metrics and support audit/replay.
• Implement a simple, extensible scheduler with weighted scoring + constraint filtering and a pluggable dispatch interface to support multiple protocols in the future.
• Ensure reliable execution with idempotency, timeout/retry and automatic fallback to secondary nodes.

Description

• Added a new pcdn service module under server/service/pcdn containing scheduler.go, policy_engine.go, dispatcher.go, fallback.go, runtime.go and enter.go implementing scheduling, scoring, abstract dispatch interface with a MockDispatcher, fallback selection and runtime task processing.
• Introduced the PcdnDispatchTask model in server/model/pcdn/dispatch_task.go to persist Top-N candidates and metrics snapshot, with task_id/trace_id uniqueness and fields for state, timeout and retry (timeout_seconds, retry_count, max_retry, next_retry_unix).
• Integrated with existing timed task system by adding server/task/pcdn_dispatch.go and registering two second-level timers in server/initialize/timer.go to run dispatch processing and status sync; registered the model for auto-migration in initialize/gorm_biz.go and added PcdnServiceGroup to service/enter.go.
• Implemented scheduler flow: filter offline/overloaded/disabled/Docker-failed nodes, preselect by region/ISP affinity, compute weighted score via PolicyEngine, return Top-N and persist PcdnDispatchTask; runtime handles dispatching, idempotent state transitions, retry/backoff and fallback.

Testing

• Formatted modified files with gofmt -w before tests.
• Ran cd server && timeout 40s go test ./model/pcdn ./service/pcdn ./task -run TestNope -count=1 which completed (packages report [no test files] / exited 0).
• Ran cd server && timeout 40s go test ./task ./initialize -run TestNope -count=1 where ./task completed (no test files) and ./initialize timed out in the environment (exit 124).
• Ran targeted go test on ./service/pcdn which returned [no test files] and exited successfully.

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