## Summary
Adds two config knobs for controlling backfill scope and neighbor graph
data retention, plus removes the dead synchronous backfill function.
## Changes
### Config knobs
#### `resolvedPath.backfillHours` (default: 24)
Controls how far back (in hours) the async backfill scans for
observations with NULL `resolved_path`. Transmissions with `first_seen`
older than this window are skipped, reducing startup time for instances
with large historical datasets.
#### `neighborGraph.maxAgeDays` (default: 30)
Controls the maximum age of `neighbor_edges` entries. Edges with
`last_seen` older than this are pruned from both SQLite and the
in-memory graph. Pruning runs on startup (after a 4-minute stagger) and
every 24 hours thereafter.
### Dead code removal
- Removed the synchronous `backfillResolvedPaths` function that was
replaced by the async version.
### Implementation details
- `backfillResolvedPathsAsync` now accepts a `backfillHours` parameter
and filters by `tx.FirstSeen`
- `NeighborGraph.PruneOlderThan(cutoff)` removes stale edges from the
in-memory graph
- `PruneNeighborEdges(conn, graph, maxAgeDays)` prunes both DB and
in-memory graph
- Periodic pruning ticker follows the same pattern as metrics pruning
(24h interval, staggered start)
- Graceful shutdown stops the edge prune ticker
### Config example
Both knobs added to `config.example.json` with `_comment` fields.
## Tests
- Config default/override tests for both knobs
- `TestGraphPruneOlderThan` — in-memory edge pruning
- `TestPruneNeighborEdgesDB` — SQLite + in-memory pruning together
- `TestBackfillRespectsHourWindow` — verifies old transmissions are
excluded by backfill window
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Co-authored-by: you <you@example.com>
