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 ($L_xapprox10^{41}~rm{erg~s^{-1}}$ 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 $sim$ $7times10^{-3}$ $rm{M_{odot}}$ with particle density $napprox10^{9},rm{cm^{-3}}$. 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 $t_r =13.4pm0.210$ days and a peak B-band magnitude of $M_B =-15.1pm0.200$ mag). We find that SN 2019ehk synthesized $(3.1pm0.11)times10^{-2} ~ rm{M_{odot}}$ of ${}^{56}textrm{Ni}$ and ejected $M_{rm ej} = (0.72pm 0.040)~rm{M_{odot}}$ total with a kinetic energy $E_{rm k}=(1.8pm0.10)times10^{50}~rm{erg}$. 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.
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