[P1][Analytics][planned] Persist session analytics summaries for trend curves

[xodapi]

Summary

Persist privacy-safe SessionAnalyticsSummaryRecord outputs from the AnalyzeSession batch contract introduced in #110 / PR #111. This is the storage bridge needed before building cognitive ability trend curves (#48) and admin analytics (#91).

Problem

#110 defines the synthetic fixtures and server-side summary contract, but summaries are not yet persisted or queryable. If #48 starts directly from UI charts, it will mix product visualization with storage decisions and make the analytics layer harder to audit.

Scope

• Add an explicit persistence contract for SessionAnalyticsSummaryRecord.
• Decide whether the first implementation is Prisma/PostgreSQL table, append-only event table, or repository abstraction over existing practice events.
• Store only privacy-safe summary fields: no raw Brain ID, no auth tokens, no email, no screenshots, no raw localStorage, no raw event payloads unless explicitly reviewed.
• Add read APIs or service functions needed by [P1][Analytics][planned] Add cognitive ability trend curves #48: by pseudonymous user/session owner, by module, by date range, and aggregate trend-ready series.
• Keep migrations small and explicit if a DB schema change is required.

Acceptance Criteria

• SessionAnalyticsSummaryRecord has a single server-side persistence owner/module.
• Writes reject identity/token/raw-storage-like material before DB persistence.
• Read contract returns trend-ready privacy-safe series for [P1][Analytics][planned] Add cognitive ability trend curves #48 without exposing raw session events.
• Tests cover create, duplicate/idempotency behavior, date-range read, module filter, and privacy rejection.
• Docs explain how this feeds [P1][Analytics][planned] Add cognitive ability trend curves #48 cognitive curves and [P2][Admin][planned] Build privacy-safe admin analytics console #91 admin analytics.
• No real user data, secrets, raw Brain ID, private telemetry, screenshots, or raw localStorage in code/tests/docs.

Verification

• pnpm lint
• pnpm test
• pnpm build
• targeted route/service tests for persistence contract

Dependencies

• Depends on [P1][Core] Golden fixtures v2 and server-side batch analytics contract #110 / PR feat: add analyze session golden fixtures v2 #111 being merged.
• Supports [P1][Analytics][planned] Add cognitive ability trend curves #48 and [P2][Admin][planned] Build privacy-safe admin analytics console #91.