FEAT GUI: Add paginated seed browsing API

Start with the dataset routes, services, models, MemoryInterface.get_seeds(), the seed/group models, and pyrit/backend/services/pagination.py; also review doc/code/framework.md and existing paging tests. Define the service and query boundaries before implementation, then use local fixtures to verify grouped pagination, search and filter semantics, previews versus full content, errors, and bounded database queries without downloads or model calls.

Is your feature request related to a problem? Please describe.

A dataset name list cannot support a scalable prompt table or a reliable selection for chat/scenarios. The GUI needs bounded retrieval, searchable metadata, and complete logical examples rather than disconnected media pieces.

Work item 2 of 17 in #2744. Technical prerequisite: item 1's dataset identity and count contract. Keep not ready yet until promoted by the maintainer.

Describe the solution you'd like

Add typed, read-only dataset browsing/detail APIs on top of existing seed memory.

• Support bounded server-side pagination with deterministic ordering and stable seed/group identifiers. A page boundary must not split a logical example.
• Support prompt-text search and existing metadata filters, including modality, harm category, and seed type. Document and test filter-combination and category-matching semantics.
• Return compact previews for listing and full content/metadata for an expanded example. Do not send media bytes in the list response.
• Preserve related pieces, roles, and sequence information. A modality match on one member must not silently remove the other members of the selected example.
• Distinguish SeedPrompt, SeedObjective, templates, and simulated-conversation configuration. Browsing must not render templates or generate conversations.
• Retain provenance and identifiers needed by later import/scenario requests. Return repository-standard errors for invalid filters, missing examples, and invalid paging inputs.

Acceptance criteria:

• Multiple pages do not duplicate or omit rows under a stable dataset, and grouped media stays together.
• Search/filter results and counts use the documented semantics.
• Full text is available even when the list preview is truncated.
• Empty results, unnamed datasets, mixed types, missing IDs, and invalid requests have deliberate behavior.
• Queries are bounded at the database layer rather than slicing a fully materialized seed list.
• Tests use local fixtures; reads perform no downloads, writes, model calls, or template execution.

Describe alternatives you've considered, if relevant

Avoid client-side paging of entire datasets, separate GUI storage, or schema migrations. Narrow memory query helpers are acceptable, but selection/presentation policy belongs in the service rather than memory.

Additional context

Start with the dataset routes/service/models, MemoryInterface.get_seeds(), the seed/group models, and pyrit\backend\services\pagination.py. Follow doc\code\framework.md and applicable database/model/test instructions. Reuse existing paging conventions where their contract fits; do not copy attack-history semantics without checking group boundaries.