docs: add stats dashboard design docs, plans, and knowledge base

- Stats dashboard redesign design and implementation plans
- Episode detail and Anki card link design
- Internal knowledge base restructure
- Backlog tasks for testing, verification, and occurrence tracking

docs/plans/2026-03-14-episode-detail-anki-link-design.md

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+# Episode Detail & Anki Card Link — Design
+
+**Date**: 2026-03-14
+**Status**: Approved
+
+## Motivation
+
+The anime detail page shows episodes and cards mined but lacks drill-down into individual episodes. Users want to see per-episode stats (sessions, words, cards) and link directly to mined Anki cards.
+
+## Design
+
+### 1. Episode Expandable Detail
+
+Click an episode row in `EpisodeList` or `AnimeCardsList` → expands inline:
+- Sessions list for this episode (sessions linked to video_id)
+- Cards mined list — timestamps + "Open in Anki" button per card (when note ID available)
+- Top words from this episode (word occurrences scoped to video_id)
+
+### 2. Anki Note ID Storage
+
+- Extend `recordCardsMined` callback to accept note IDs: `recordCardsMined(count, noteIds)`
+- Store in CARD_MINED event payload: `{ cardsMined: 1, noteIds: [12345] }`
+- Proxy already has note IDs in `pendingNoteIds` — pass through callback chain
+- Polling has note IDs from `newNoteIds` — same treatment
+- No schema change — note IDs stored in existing `payload_json` column on `imm_session_events`
+
+### 3. "Open in Anki" Flow
+
+- New endpoint: `POST /api/stats/anki/browse?noteId=12345`
+- Calls AnkiConnect `guiBrowse` with query `nid:12345`
+- Opens Anki's card browser filtered to that note
+- Frontend button hits this endpoint
+
+### 4. Episode Words
+
+- New query: `getEpisodeWords(videoId)` — like `getAnimeWords` but filtered by video_id
+- Reuse AnimeWordList component pattern
+
+### 5. Backend Changes
+
+**Modified files:**
+- `src/anki-integration/anki-connect-proxy.ts` — pass note IDs through recordCardsAdded callback
+- `src/anki-integration/polling.ts` — pass note IDs through recordCardsAdded callback
+- `src/anki-integration.ts` — update callback signature
+- `src/core/services/immersion-tracker-service.ts` — accept and store note IDs in recordCardsMined
+- `src/core/services/immersion-tracker/query.ts` — add getEpisodeWords, getEpisodeSessions, getEpisodeCardEvents
+- `src/core/services/stats-server.ts` — add episode detail and anki browse endpoints
+
+### 6. Frontend Changes
+
+**Modified files:**
+- `stats/src/components/anime/EpisodeList.tsx` — make rows expandable
+- `stats/src/components/anime/AnimeCardsList.tsx` — make rows expandable
+
+**New files:**
+- `stats/src/components/anime/EpisodeDetail.tsx` — inline expandable content