refactor: centralize all physical constants in constants.py, fix unit naming

- Add EULER_GAMMA, ME_KEV, R0_FM to constants.py
- Replace hardcoded 0.511, 0.5772..., 1.2 with imports from constants
- Fix T_to_W parameter name: T_kev -> T_MeV (it takes MeV, not keV)
- Remove duplicate ME_ME from spectrum.py (import ME_MEV from constants)
- Remove hardcoded r_0 from detector_response.py and radiative.py
- Update CONVENTIONS.md to reference correct constant names

beta_spectrum/spectrum.py

@@ -13,12 +13,11 @@ from beta_spectrum.components.exchange import ExchangeCorrection
 from beta_spectrum.components.radiative import RadiativeCorrection
 
 from beta_spectrum.utils import T_to_W
+from beta_spectrum.constants import ME_MEV
 
 if TYPE_CHECKING:
     from beta_spectrum.components.detector_response import DetectorResponse
 
-ME_ME = 0.510998950  # electron rest mass in MeV
-
 
 @dataclass
 class SpectrumConfig:
@@ -165,8 +164,9 @@ class BetaSpectrum:
         from beta_spectrum.components.detector_response import DetectorResponse
         from beta_spectrum.utils import T_to_W
 
-        sigma_a_me = config.detector_sigma_a_keV / ME_ME
-        tau_me = config.detector_tau_keV / ME_ME
+        sigma_a_me = config.detector_sigma_a_keV / ME_MEV
+
+        tau_me = config.detector_tau_keV / ME_MEV
 
         W0 = T_to_W(config.endpoint_MeV)
         channel_range = config.detector_channel_energy_range