Add design spec for incremental snapshot downloader

docs/superpowers/specs/2026-05-15-incremental-snapshot-design.md

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+# Incremental Snapshot Downloader Design
+
+**Date:** 2026-05-15
+**Status:** Approved
+
+## Problem
+
+The snapshot downloader currently produces a new HTML file on every daily run, named `snapshots_{date_from}_to_{date_to}.html`. Because `date_to` changes each day, files accumulate with near-identical content. The user wants a single file updated in place each day.
+
+## Approach
+
+Incremental fetch with a JSON data cache and a state file. On each run, only new snapshots are fetched from the API; they are merged into a persistent cache of all snapshot objects; the single output HTML is re-rendered from the full cache.
+
+## Data Files
+
+All written to `output_dir`:
+
+| File | Purpose |
+|---|---|
+| `snapshots_cache.json` | Array of every snapshot object fetched so far |
+| `last_run.json` | `{"last_date_to": "YYYY-MM-DD"}` — upper bound of the last successful fetch |
+| `snapshots.html` | Single output file, fixed name, overwritten each run |
+
+## Run Logic
+
+1. Read `last_run.json` → use `last_date_to` as `date_from` for this run. Fall back to the `date_from` argument (from config) if no state file exists yet (first run).
+2. Load `snapshots_cache.json` if it exists → existing snapshot list. Empty list if absent.
+3. Fetch new snapshots from the API with `date_from` (from step 1) and `date_to = today`.
+4. Merge: append new snapshots to the cache list, deduplicating by snapshot `id`. The boundary date may be returned by the API again on subsequent runs.
+5. Save the updated `snapshots_cache.json`.
+6. Re-render `snapshots.html` from the full merged list (sorted newest-first, existing logic unchanged).
+7. Write `last_run.json` with today's date.
+
+**On fetch failure:** steps 5–7 are skipped. The cache and state file remain unchanged, so the next run retries from the same `date_from`.
+
+## Code Changes
+
+All changes are in `src/snapshot_downloader.py`. `ConfigSnapshotDownloader` is unchanged.
+
+### New methods on `SnapshotDownloader`
+
+- `load_snapshot_cache() -> List[Dict]` — reads `snapshots_cache.json`; returns `[]` if absent or malformed
+- `save_snapshot_cache(snapshots: List[Dict])` — writes `snapshots_cache.json`
+- `load_last_run_date() -> Optional[str]` — reads `last_run.json`; returns `None` if absent
+- `save_last_run_date(date: str)` — writes `last_run.json`
+
+### Modified methods
+
+**`download_snapshots(type_ids, date_from, date_to, max_pages)`**
+
+`date_from` becomes the "initial history start" fallback (used only when `last_run.json` does not exist). `date_to` is always set to today's date, regardless of what is passed. The method orchestrates the 7-step incremental flow above.
+
+**`generate_html_file(snapshots, date_from, date_to)`**
+
+Output filename changes from `snapshots_{date_from}_to_{date_to}.html` to the fixed name `snapshots.html`.
+
+## Out of Scope
+
+- Detecting and applying updates to existing snapshot objects (notes edits, etc.) — snapshots are treated as append-only
+- Pruning the cache (old entries are kept indefinitely)
+- Changes to `ConfigSnapshotDownloader`, the web server, or any other module