refactor(release): make tech_string a derived property

ParsedRelease.tech_string was a stored str field re-computed in two
places (assemble() at parse time, enrich_from_probe() after the probe).
The second site was a reactive fix (e79ca46) for filename builders that
saw a stale value. Turn it into an @property so it stays in sync with
quality/source/codec by construction.

- Drop the field from the dataclass + the key from assemble()'s dict.
- Drop tech_string="" from parse_release's malformed-name fallback.
- Drop the manual recomputation at the end of enrich_from_probe.
- Inject the property into asdict() result in the fixtures runner
  (same treatment as is_season_pack).
- Update tests that passed tech_string= to the constructor; rewrite the
  TestTechString case that mutated p.tech_string manually.

tests/domain/release/test_parser_v2_easy.py

@@ -123,7 +123,6 @@ class TestAssemble:
         assert fields["source"] == "WEBRip"
         assert fields["codec"] == "x265"
         assert fields["group"] == "KONTRAST"
-        assert fields["tech_string"] == "1080p.WEBRip.x265"
         assert fields["media_type"] == "movie"
         assert fields["site_tag"] is None
 
@@ -150,7 +149,8 @@ class TestAssemble:
         assert fields["season"] == 2
         assert fields["episode"] is None  # season pack
         assert fields["source"] is None  # ELiTE omits it
-        assert fields["tech_string"] == "1080p.x265"
+        assert fields["quality"] == "1080p"
+        assert fields["codec"] == "x265"
         assert fields["group"] == "ELiTE"