  We present panchromatic observations and modeling of the Calcium-rich
supernova 2019ehk in the star-forming galaxy M100 (d$\approx$16.2 Mpc) starting
10 hours after explosion and continuing for ~300 days. SN 2019ehk shows a
double-peaked optical light curve peaking at $t = 3$ and $15$ days. The first
peak is coincident with luminous, rapidly decaying $\textit{Swift}$-XRT
discovered X-ray emission ($L_x\approx10^{41}~\rm{erg~s^{-1}}$ at 3 days; $L_x
\propto t^{-3}$), and a Shane/Kast spectral detection of narrow H$\alpha$ and
He II emission lines ($v \approx 500$ km/s) originating from pre-existent
circumstellar material. We attribute this phenomenology to radiation from shock
interaction with extended, dense material surrounding the progenitor star at
$r<10^{15}$ cm and the resulting cooling emission. We calculate a total CSM
mass of $\sim$ $7\times10^{-3}$ $\rm{M_{\odot}}$ with particle density
$n\approx10^{9}\,\rm{cm^{-3}}$. Radio observations indicate a significantly
lower density $n < 10^{4}\,\rm{cm^{-3}}$ at larger radii. The photometric and
spectroscopic properties during the second light curve peak are consistent with
those of Ca-rich transients (rise-time of $t_r =13.4\pm0.210$ days and a peak
B-band magnitude of $M_B =-15.1\pm0.200$ mag). We find that SN 2019ehk
synthesized $(3.1\pm0.11)\times10^{-2} ~ \rm{M_{\odot}}$ of
${}^{56}\textrm{Ni}$ and ejected $M_{\rm ej} = (0.72\pm 0.040)~\rm{M_{\odot}}$
total with a kinetic energy $E_{\rm k}=(1.8\pm0.10)\times10^{50}~\rm{erg}$.
Finally, deep $\textit{HST}$ pre-explosion imaging at the SN site constrains
the parameter space of viable stellar progenitors to massive stars in the
lowest mass bin (~10 $\rm{M_{\odot}}$) in binaries that lost most of their He
envelope or white dwarfs. The explosion and environment properties of SN
2019ehk further restrict the potential WD progenitor systems to low-mass hybrid
HeCO WD + CO WD binaries.
