Add Grafana dashboard dependency and portability manifest

### Problem\n\nThe Grafana dashboard catalog can view or export JSON, but operators planning an upgrade or migration must manually inspect every model to learn which panel types, data sources, template variables, query targets, transformations, library panels, repeats, and embedded alerts are required. Structural omissions are easy to miss.\n\n### Proposed solution\n\nAdd an on-demand, read-only dependency manifest that loads dashboard models in bounded batches, recursively analyzes nested panels, summarizes portfolio dependencies, flags structural portability notes, supports search, and exports the current result as CSV.\n\nThis does not mutate dashboards or introduce a new backend contract.

No files, tests, or entry points are named in the issue. Start by locating the Grafana dashboard catalog and its dashboard-model loading and export paths, then assess where a read-only, bounded-batch dependency manifest fits. Done means the requested dependency categories, portability notes, search, and CSV export are covered without mutating dashboards or adding a backend contract.

Written by the indexing model from the issue text.