fix(release): refresh tech_string after enrich_from_probe

enrich_from_probe fills None fields on ParsedRelease (quality, source,
codec, audio_*, languages) but left tech_string at its parser-time
value — so the filename builders (movie_folder_name, episode_filename,
…) saw stale tech tokens even after a successful probe.

Re-derive tech_string the same way the parser does — quality.source.codec
joined by dots, skipping None — at the end of enrich_from_probe. Token-
level values still win because enrich only fills None fields.

Four new tests in TestTechString cover: enrichment rebuilds it,
existing source survives, no-info input leaves it untouched, fully
empty parsed produces ''.

tests/application/test_enrich_from_probe.py

@@ -210,3 +210,42 @@ class TestLanguages:
         p = _bare()
         enrich_from_probe(p, MediaInfo())
         assert p.languages == []
+
+
+# --------------------------------------------------------------------------- #
+# tech_string                                                                  #
+# --------------------------------------------------------------------------- #
+
+
+class TestTechString:
+    """tech_string drives the filename builders; it must be re-derived
+    whenever quality / source / codec change."""
+
+    def test_rebuilt_from_filled_quality_and_codec(self):
+        p = _bare()
+        enrich_from_probe(
+            p, _info_with_video(width=1920, height=1080, codec="hevc")
+        )
+        assert p.quality == "1080p"
+        assert p.codec == "x265"
+        assert p.tech_string == "1080p.x265"
+
+    def test_keeps_existing_source_when_enriching(self):
+        # Token-level source must stay; probe fills only None fields.
+        p = _bare(source="BluRay")
+        enrich_from_probe(
+            p, _info_with_video(width=1920, height=1080, codec="hevc")
+        )
+        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.
+        p = _bare(quality="2160p", source="WEB-DL", codec="x265")
+        p.tech_string = "2160p.WEB-DL.x265"
+        enrich_from_probe(p, MediaInfo())
+        assert p.tech_string == "2160p.WEB-DL.x265"
+
+    def test_empty_when_nothing_known(self):
+        p = _bare()
+        enrich_from_probe(p, MediaInfo())
+        assert p.tech_string == ""