RadarStore: use mounted data_dir so deploys don't wipe radars

[fleveque]

Bug

After the radar PR shipped, every Pulse deploy wiped users' radar listings — they had to re-opt-in (re-toggle Share Radar in Quantic, or wait for the next `radar.updated` event) to see their radar reappear. Portfolios survived deploys fine.

Root cause

RadarStore was the odd one out among DETS-using modules: it called `:code.priv_dir(:pulse)` directly to locate `radars.dets`, which resolves inside the release directory. Kamal rebuilds the release per image, so the file was always empty after a deploy.

Every other DETS user (`Pulse.Store`, `Pulse.Analytics`, `Pulse.RadarAnalytics`) reads `data_dir` from app env, which `config/runtime.exs` overrides via the `DATA_DIR` env var → `/app/data` in prod, a Kamal-mounted volume that persists across deploys.

Fix

Make RadarStore.init/1 read the configurable `data_dir` and join `radars.dets` onto it. One-line equivalence with the other modules.

• Prod: resolves to `/app/data/radars.dets` (mounted volume — survives deploys).
• Dev: stays at `priv/data/radars.dets` (default config).
• Test: `config/test.exs` already overrides `data_dir` to `priv/data/test`.

Migration note for this deploy

This deploy will start with an empty `/app/data/radars.dets` because the file was never written to that location before. The radar workers currently in memory (re-opted-in after the last deploy) live only in the old `priv/data/` path inside the running container, which goes away when the new image takes over.

Two options to preserve current radar state across this deploy:

1. Easiest — accept one more re-opt cycle: deploy this PR, then ask affected users to toggle Share Radar off/on once. From then on, radars survive every future deploy.
2. Manual save — before the new container takes over, copy the in-container file to the volume:
   ```
   kamal app exec --reuse -- cp /app/lib/pulse-/priv/data/radars.dets /app/data/radars.dets
   ```
   (Path inside the release tree — check with `kamal app exec --reuse -- find /app -name radars.dets` first.)

Test plan

• mix test — 44 / 44
• mix format --check-formatted — clean
• mix compile --warnings-as-errors — clean
• Manual after deploy: verify `/app/data/radars.dets` exists and grows as users opt in
• Manual: kill + restart the Pulse container, confirm `RadarStore.restore_all` restores the workers

🤖 Generated with Claude Code