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/application/test_enrich_from_probe.py

@@ -46,7 +46,6 @@ def _bare(**overrides) -> ParsedRelease:
         source=None,
         codec=None,
         group="UNKNOWN",
-        tech_string="",
     )
     defaults.update(overrides)
     return ParsedRelease(**defaults)
@@ -218,8 +217,9 @@ class TestLanguages:
 
 
 class TestTechString:
-    """tech_string drives the filename builders; it must be re-derived
-    whenever quality / source / codec change."""
+    """tech_string is a derived property on ParsedRelease: it always
+    reflects the current quality/source/codec. Enrichment never writes
+    it directly — it stays in sync by construction."""
 
     def test_rebuilt_from_filled_quality_and_codec(self):
         p = _bare()
@@ -239,9 +239,9 @@ class TestTechString:
         assert p.tech_string == "1080p.BluRay.x265"
 
     def test_unchanged_when_no_enrichable_video_info(self):
-        # No video info → nothing to fill → tech_string stays as it was.
+        # No video info → nothing to fill → derived tech_string stays as it was.
         p = _bare(quality="2160p", source="WEB-DL", codec="x265")
-        p.tech_string = "2160p.WEB-DL.x265"
+        assert p.tech_string == "2160p.WEB-DL.x265"
         enrich_from_probe(p, MediaInfo())
         assert p.tech_string == "2160p.WEB-DL.x265"