feat: "Not a dupe" ignore with SQLite persistence

- New ignore_db module with SQLite-backed dismissal storage
- Groups flagged as not-a-dupe are persisted to ~/.config/deduper/ignores.db
- Fingerprint based on sorted SHA256 hashes (content-stable)
- Ignored groups filtered out on subsequent runs
- Review UI: green "Not a dupe" button per group
- Dismissed groups fade out immediately in browser
- DEDUPER_DB_DIR env var to override DB location
- 4 new unit tests for ignore_db
- 29 tests passing

Cargo.toml

@@ -13,3 +13,4 @@ open = "5"
 base64 = "0.22"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+rusqlite = { version = "0.31", features = ["bundled"] }