Add KigamQuake waveform source for recent KG events missing from KMA NECIS

[seismoseo]

Motivation

PocketQuake fetches event waveforms from KMA NECIS (default) and STP (--source stp / mixed). But recent KG-network (KIGAM) events are often not in NECIS — in the Namwon test, 5 of 12 catalog events had no NECIS match within tolerance (closest NECIS event 17–68 min away) and were skipped. KIGAM serves these through its own platform, KIGAM Quake.

Source: KIGAM Quake

• Platform / web services: https://data.kigam.re.kr/quake/data/web-services (KIGAM Seismic Network — broadband + strong-motion + infrasound). Check whether it exposes FDSN-standard dataselect/station services — if so, ObsPy's FDSN client could fetch directly with minimal new code. (FDSN data-center registry: https://www.fdsn.org/citations/)
• Reference: KIGAM Quake: An open platform for seismological data and earthquake research information, Geomechanics and Engineering (Techno-Press), Vol. 37, No. 3. https://www.techno-press.org/content/?page=article&journal=gae&volume=37&num=3&ordernum=7

Scope (rough)

• New waveform-source backend alongside necis / stp (mirrors necis-downloader / the STP bridge): per-event fetch → SAC into the cluster's waveform tree, same downstream format.
• Wire --source kigam, and consider per-event mixed dispatch (NECIS / STP / KIGAM by availability).
• Credentials/endpoint in .env (like NECIS_USER / STP); verify the exact API + access terms.
• Document the network/coverage (recent KG) in EXTERNAL_TOOLS / INSTALL.

Out of scope

Not implementing now — tracking only.

— filed via Claude Code