Wire up p4n2gpx as a private plugin

data/plugins/p4n2gpx/ (outside git, see .gitignore) now holds a
plugin.json + run.rb wrapper around ../p4n2gpx/ruby/p4n2gpx.rb, so it
runs through the same --south/--west/--north/--east-in,
GeoJSON-on-stdout-out contract as the built-in overpass-pois plugin.
run.rb converts the requested bbox to the center+radius p4n2gpx.rb
itself expects, then reuses lib/maptool/gpx.rb to turn its .gpx output
into GeoJSON.

p4n2gpx.rb needs faraday-cookie_jar, which wasn't in the Gemfile yet
(faraday itself already was, transitively via oauth2) — added and
locked. Also wires P4N_USER/P4N_PASSWORD through docker-compose.yaml
and the deploy workflow's "Write .env" step, the same optional-secret
pattern as LIVETRACK_WEBHOOK_SECRET, since the container has no
browser for p4n2gpx.rb's Chrome-cookie auth fallback.

Gemfile.lock

@@ -7,11 +7,15 @@ GEM
       version_gem (~> 1.1, >= 1.1.14)
     base64 (0.3.0)
     bigdecimal (4.1.2)
+    domain_name (0.6.20260902)
     drb (2.2.3)
     faraday (2.14.3)
       faraday-net_http (>= 2.0, < 3.5)
       json
       logger
+    faraday-cookie_jar (0.0.8)
+      faraday (>= 0.8.0)
+      http-cookie (>= 1.0.0)
     faraday-net_http (3.4.4)
       net-http (~> 0.5)
     ffi (1.17.4-aarch64-linux-gnu)
@@ -26,6 +30,8 @@ GEM
     ffi (1.17.4-x86_64-linux-musl)
     hashie (5.1.0)
       logger
+    http-cookie (1.1.6)
+      domain_name (~> 0.5)
     json (2.21.2)
     jwt (3.2.0)
       base64
@@ -106,6 +112,7 @@ PLATFORMS
   x86_64-linux-musl
 
 DEPENDENCIES
+  faraday-cookie_jar (~> 0.0.8)
   minitest (~> 6.0)
   oauth2 (~> 2.0)
   puma (~> 8.0)
@@ -122,8 +129,10 @@ CHECKSUMS
   base64 (0.3.0) sha256=27337aeabad6ffae05c265c450490628ef3ebd4b67be58257393227588f5a97b
   bigdecimal (4.1.2) sha256=53d217666027eab4280346fba98e7d5b66baaae1b9c3c1c0ffe89d48188a3fbd
   bundler (4.0.18) sha256=02d9a17429de1847b4e0c9f27a9ee4b20c0a74c0a641b4e77195d6019e3618ac
+  domain_name (0.6.20260902) sha256=4fac7f8e0a77dc85ec02f6534fdeff38e143d434ab8a86528fe68ca5bc45a042
   drb (2.2.3) sha256=0b00d6fdb50995fe4a45dea13663493c841112e4068656854646f418fda13373
   faraday (2.14.3) sha256=1882247e6766615c8220b4392bf1d27f6ebb63d8e28267587cef1fb0bf37f278
+  faraday-cookie_jar (0.0.8) sha256=0140605823f8cc63c7028fccee486aaed8e54835c360cffc1f7c8c07c4299dbb
   faraday-net_http (3.4.4) sha256=0e78af151747ed1b00f33e25973b4bc220d7f16c00c39676817c8b12331eb588
   ffi (1.17.4-aarch64-linux-gnu) sha256=b208f06f91ffd8f5e1193da3cae3d2ccfc27fc36fba577baf698d26d91c080df
   ffi (1.17.4-aarch64-linux-musl) sha256=9286b7a615f2676245283aef0a0a3b475ae3aae2bb5448baace630bb77b91f39
@@ -136,6 +145,7 @@ CHECKSUMS
   ffi (1.17.4-x86_64-linux-gnu) sha256=9d3db14c2eae074b382fa9c083fe95aec6e0a1451da249eab096c34002bc752d
   ffi (1.17.4-x86_64-linux-musl) sha256=3fdf9888483de005f8ef8d1cf2d3b20d86626af206cbf780f6a6a12439a9c49e
   hashie (5.1.0) sha256=c266471896f323c446ea8207f8ffac985d2718df0a0ba98651a3057096ca3870
+  http-cookie (1.1.6) sha256=ba4b82be64de61dc281243dac70e3c382c45142f20268ed9276a3670c93feaa9
   json (2.21.2) sha256=1f1d3b7cf2b3ba1a69beca0bb6db13d5438b80bff3cd54cdaaa620b9b07c1c6a
   jwt (3.2.0) sha256=5419b1fe37b1da0982bd07051f573a8b8789ab724c2aa7e785e4784a3ed217d7
   listen (3.10.0) sha256=c6e182db62143aeccc2e1960033bebe7445309c7272061979bb098d03760c9d2