feat(learning): persist mastery prerequisites and topic-source links

Background

Discussion #1290 proposes Mastery-gated progressive knowledge bases / Learn-Answer modes. Today a KnowledgePoint stores only its identity, name, type, and module. It cannot express which objectives are required before an objective, nor which selected topic sources justify that objective.

First-slice proposal

Add durable structural metadata to mastery objectives:

• prerequisite_ids: required prerequisite objectives, suitable for a future hard eligibility gate.
• topic_source_ids: selected non-goal topic sources that justify the objective.

Keep this first slice to the structural foundation:

• Add both fields with empty defaults so legacy paths require no migration.
• Resolve request-local client_ref / relation refs and persistent ids inside the mastery service transaction.
• Remap prerequisite refs after final module/KP ids are allocated, including append renumbering and revise rewrites.
• Validate that prerequisite targets exist on the same path; there are no self references or cycles; topic sources exist on the current topic and are not the goal source; and duplicate references are removed while preserving first occurrence.
• Wire mastery_build, mastery_revise, topic create/edit, map payloads, and lossless frontend round-tripping.

Non-goals for this slice

• No automatic relationship generation in topic drafts.
• No retrieval restriction, mastery gate, or Learn-Answer behavior change.
• No visual relationship editor.
• No claim that source links prove factual correctness; access and availability checks remain downstream concerns.

Acceptance

• Legacy paths load with empty relation lists.
• A failed relation validation rolls back the whole mutation.
• Replace creates fresh objective identities rather than inheriting evidence by positional id collision.
• Append renumbering leaves no dangling prerequisites.
• Rewriting an objective that is referenced across modules remaps every incoming edge.
• Removing an objective still referenced by others reports its dependents.
• Renaming or reordering objectives in the route editor preserves all relations.

Refs #1290