[Retroactive M3] Backend + Frontend: manual note markers

[Sturmi77]

Context

Part of the Notes in Analysis Epic. M3 (Insights v1) is already completed. This issue retroactively adds manual note markers — the safest bridge from raw free-text to explainable analytics, fully within the existing API-first architecture.

Depends on: #195 (backend note fields), #196 (note visibility)

Spec Reference

docs/features/notes-in-analysis.md — Sections: Marker Taxonomy v1, Frontend Components — Entry Composer

Marker Taxonomy v1

Key	Display (DE/EN)
work	Arbeit / Work
homeoffice	Homeoffice / Remote
social	Sozial / Social
movement	Bewegung / Exercise
sleep_bad	Schlechter Schlaf / Poor Sleep
sleep_good	Guter Schlaf / Good Sleep
stress	Stress
conflict	Konflikt / Conflict
symptom	Symptom
travel	Reise / Travel
achievement	Erfolg / Achievement

Custom markers: free-text, max 32 chars.

Tasks

Backend

• POST /entries/{id}/note-markers with validation against taxonomy + custom rules
• DELETE /entries/{id}/note-markers/{marker_id}
• GET /users/me/note-markers/suggestions (most-used first, max 20)
• GET /analysis/notes/marker-summary endpoint (see spec for response shape)
• ADR-N-03: decide if custom markers are normalised server-side or stored verbatim — file ADR before implementation

Frontend

• Chip row in Entry Composer showing 11 predefined markers + last 5 user-defined
• Multi-select, persists to entry_note_markers with source: 'user'
• Chip row scrolls horizontally on mobile (375px), no wrapping
• Filter chip: "Nur markierte Einträge" + filter by specific marker
• Markers visible as read-only chips in Entry Drawer (from [Retroactive M2] Frontend: note visibility in timeline, calendar, and analysis drilldown #196)

Acceptance Criteria

• Entry Composer shows 11 predefined marker chips
• Selected markers saved and synced correctly
• Entries can be filtered by marker in analysis view
• GET /analysis/notes/marker-summary returns correct avg_mood per marker
• Suggestions endpoint returns last 20 user-defined markers
• Mobile chip row works at 375px
• ADR-N-03 filed before merge