feat(server): add PCDN data models and auto-migrate registration

[hequan2017]

Motivation

• Introduce PCDN domain entities to model nodes, resources, scheduling policies, dispatch tasks and realtime metrics for upcoming CDN scheduling features.
• Ensure these new tables are created during the existing GORM biz migration flow and provide request/response DTOs for API integrations.

Description

• Added new model definitions in server/model/pcdn/pcdn.go for PcdnNode, PcdnResource, PcdnPolicy, PcdnDispatchTask, and PcdnMetricSnapshot with table names, field tags and constraints.
• Created search/request DTOs in server/model/request/pcdn.go and list response DTOs in server/model/response/pcdn.go for paging/filtering of each entity.
• Registered the five PCDN models in the business AutoMigrate flow inside bizModel() in server/initialize/gorm_biz.go so they are included in GORM migrations.
• Added useful indexes and uniqueness keys: idx_pcdn_node_region_isp (region+isp), idx_pcdn_metric_node_time (node_id+snapshot_at), idx_pcdn_dispatch_status_created (status+created_at_unix), plus unique/index keys for node/task/policy identity fields.
• Formatted new files with gofmt.

Testing

• Ran gofmt on modified files to ensure formatting (succeeded).
• Attempted to run go test for the affected packages under server/ but go test in this environment timed out; no package-level unit tests were added for these new models in this change. The migration registration change was validated by compiling/formatting and by inspecting bizModel() to confirm the new types are included.

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