[verified] Add per-symbol analysis breakdown endpoint + per-theme narrative

- themes.symbol_breakdown(): transparent scoring derivation (theme_score, siamchart_score components, combined = 0.6*theme + 0.4*siamchart npolut)
- GET /api/v1/symbols/<symbol>: themes + theme surprise contributions + fundamentals + price + weights (ข้อ 7)
- dashboard.py _theme_narrative(): long-form Thai explanation of each theme's analysis outcome + implication for its stocks (ข้อ 5)
- 2 tests; full suite OK; live verified (AOT: combined 0.107 = 0.6*0.571 + 0.4*(-0.588))

backend/tests/test_themes.py

@@ -70,3 +70,36 @@ class ThemesTest(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class SymbolBreakdownTest(unittest.TestCase):
+    def test_breakdown_shows_components(self):
+        factor_view = {
+            "factors": [
+                {"symbol": "AOT", "eps_growth_yoy": 10.0, "dividend_yield": 2.0,
+                 "pe": 20.0, "eps": 5.0, "pbv": 2.0, "roe": 15.0, "is_dividend": True,
+                 "company_name": "Airports"},
+            ]
+        }
+        theme_surprises = {"tourism": 0.57, "auto_credit": 0.81, "refining_energy": 1.62}
+        from app import themes
+        d = themes.symbol_breakdown("AOT", factor_view=factor_view, theme_surprises=theme_surprises,
+                                    latest_price=67.0, price_date="2026-08-21")
+        self.assertEqual(d["symbol"], "AOT")
+        self.assertEqual(d["themes"], ["tourism"])  # AOT in tourism map
+        self.assertEqual(d["theme_contributions"][0]["surprise"], 0.57)
+        self.assertIn("siamchart_components", d)
+        self.assertEqual(d["weights"], {"theme": 0.6, "siamchart": 0.4})
+        self.assertIsInstance(d["combined_score"], float)
+        self.assertEqual(d["price"]["latest"], 67.0)
+        self.assertIn("fundamentals", d)
+
+    def test_breakdown_symbol_without_theme(self):
+        factor_view = {"factors": [{"symbol": "BANPU", "eps_growth_yoy": -2.0,
+                                    "dividend_yield": 0.0, "is_dividend": False,
+                                    "company_name": "BANPU"}]}
+        from app import themes
+        d = themes.symbol_breakdown("BANPU", factor_view=factor_view, theme_surprises={})
+        self.assertEqual(d["theme_score"], 0.0)
+        self.assertEqual(d["themes"], ["refining_energy"])  # BANPU in energy map
+        self.assertEqual(d["theme_contributions"], [])     # but no surprise set -> 0