docs: update docs, add backlog tasks and change notes

Update configuration, immersion tracking, and mining workflow docs.
Add backlog tasks for upcoming work items and change notes for recent
features and fixes.

docs/architecture/stats-trends-data-flow.md

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+# Stats Trends Data Flow
+
+read_when: touching stats trend charts, changing stats API payloads, or debugging dashboard performance
+
+## Summary
+
+Trend charts now consume one chart-oriented backend payload from `/api/stats/trends/dashboard`.
+
+## Why
+
+- remove repeated client-side dataset rebuilding in `TrendsTab`
+- collapse multiple network round-trips into one request
+- keep heavy chart shaping close to tracker/query logic
+
+## Data Sources
+
+- rollup-backed:
+  - activity charts
+  - cumulative watch/cards/words/sessions trends
+  - per-anime watch/cards/words/episodes series
+- session-metric-backed:
+  - lookup trends
+  - lookup rate trends
+  - watch-time by day-of-week/hour
+- vocabulary-backed:
+  - new-words trend
+
+## Contract
+
+The stats UI should treat the trends payload as chart-ready data. Presentation-only work in the client is fine, but rebuilding the main trend datasets from raw sessions should stay out of the render path.