feat: add FRANCIS_HOST to connect to a standalone Francis runtime

FRANCIS_HOST decides where the Francis actor runtime lives. When it is
empty or set to "embedded" (the default), Pocket ID starts the runtime inside its own process, backed by its own database.

Any other value is the address, or a comma-separated list of addresses, of a standalone Francis runtime. Pocket ID then connects to it as a remote actor host and starts no embedded runtime.

Note: connecting to a standalone runtime also needs FRANCIS_HOST_PSK or FRANCIS_HOST_JWT, and optionally (but recommended) FRANCIS_CA.

tests/setup/docker-compose-francis.yml

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+# This Docker Compose file is used to set up the environment for the tests.
+# It's the variant where Pocket ID does not embed the Francis actor runtime, but connects to a standalone one instead.
+services:
+  lldap:
+    extends:
+      file: docker-compose.yml
+      service: lldap
+  scim-test-server:
+    extends:
+      file: docker-compose.yml
+      service: scim-test-server
+  francis-runtime:
+    image: ghcr.io/italypaleale/francis:0.1.0-rc.2
+    volumes:
+      - ./francis-config.yaml:/etc/francis/config.yaml:ro
+      - francis-test-data:/data
+    # The image ships its own HEALTHCHECK, which probes the runtime over the loopback
+    # It's repeated here so Pocket ID can wait on it, and so a runtime that never comes up fails fast instead of after the default retries
+    healthcheck:
+      test: ["CMD", "/bin/francis", "healthcheck"]
+      interval: 2s
+      timeout: 5s
+      retries: 15
+      start_period: 5s
+  pocket-id:
+    extends:
+      file: docker-compose.yml
+      service: pocket-id
+    environment:
+      APP_ENV: test
+      ENCRYPTION_KEY: test-encryption-key
+      FILE_BACKEND: ${FILE_BACKEND}
+      # The runtime's port is UDP, since WebTransport runs over HTTP/3
+      FRANCIS_HOST: francis-runtime:7400
+      # Must match "bootstrap.hostPSK" in francis-config.yaml
+      FRANCIS_HOST_PSK: e2e-host-bootstrap-psk-0123456789
+      # FRANCIS_CA is intentionally unset, so this exercises the same trust-on-first-use path an operator gets without it
+      # The cluster only exists inside this Compose network for the duration of the tests
+      #
+      # Peers reach actors placed on this host at ACTORS_HOST, which the runtime hands out, so it has to be the address other containers resolve rather than the default wildcard
+      ACTORS_HOST: pocket-id
+    depends_on:
+      francis-runtime:
+        condition: service_healthy
+
+volumes:
+  pocket-id-test-data:
+  francis-test-data:

tests/setup/francis-config.yaml

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+# Configuration for the standalone Francis runtime used by the "remote Francis" E2E variant.
+# In that variant Pocket ID does not embed the actor runtime: it connects to this one instead, which owns the actor state, placement, and alarms.
+# These secrets are fixed test values and must match the FRANCIS_HOST_PSK passed to Pocket ID in docker-compose-francis.yml.
+
+# The WebTransport server runs over HTTP/3, so this port is UDP
+bind: "0.0.0.0:7400"
+
+# The runtime PSKs derive the cluster CA that signs every workload certificate
+runtimePSKs:
+  - "e2e-runtime-psk-0123456789abcdef"
+
+# Hosts prove they may join by presenting this pre-shared key
+bootstrap:
+  method: psk
+  hostPSK: "e2e-host-bootstrap-psk-0123456789"
+
+# The runtime owns its own SQLite store, which is separate from Pocket ID's database
+# It lives on a volume because the image runs as a non-root user that cannot write to the image filesystem
+provider:
+  connectionString: "/data/francis.db"
+
+# A single Pocket ID replica joins the cluster, matching the cap the embedded runtime applies when HA is off
+maxHosts: 1
+
+log:
+  level: debug