SECOND EDIT: SOLVED. See bottom. But I’ll leave this up for future googlers.
EDIT: I just tried doing the TEN second hold of the setup button and then re-adding the device in the Photon app - now it starts up, flashes green, flashes cyan, then blinks RED once, then flashes cyan, and endlessly repeats this cycle of flashing CYAN with one red blink in between. This happens whether I try to start it in safe mode or not. I have no idea what’s going on at this point, but I’m pretty sure the wifi chip isn’t fried since I can at least talk to it with the app.
This is where I’m at now:
The power went out at my house for about 2 hours. When it came back on, ONE of my photon’s wouldn’t reconnect (I have one indoors one outdoors acting as weather sensors - the one outdoors reconnected just fine, but the indoor one refuses to). It just sits there flashing blue (dark blue, not cyan). I tried holding down the setup button for 3 seconds to put it in listening mode, then I launched the app on my phone, added a new device (since it just refused to see the existing already named device), it found the Photon’s wifi hotspot, I told it what my home’s wifi SSID and password is, then I saw the photon flash green…
…aaaand back to flashing blue. I’m really at a loss here as to why this would happen at all, any suggestions as to what I could do? It’s been working fine for months. Pretty sure it isn’t a power surge issue, every other device in my house is fine, and after all it was being powered by a wall wart USB adapter, not straight 110v.