Make docker-compose.yml enforce the api/enterprise-api binary swap

Closes the local/dev half of a gap named repeatedly across this
phase's docs: Helm already made api/enterprise-api mutually exclusive
(same enterprise.enabled flag that turns on RBAC/audit/SSO, both
rendering to the same Service name/port); docker-compose.yml let both
run side by side, with nothing actually pointing at enterprise-api by
default.

Mechanism: both services now carry a `profiles` entry (single-tenant /
enterprise), selected via COMPOSE_PROFILES -- a new checked-in .env
sets single-tenant as the zero-config default (unchanged behavior for
anyone who doesn't touch it), and `COMPOSE_PROFILES=enterprise docker
compose up` swaps to enterprise-api instead. Docker Compose profiles
are purely additive (no "profile X excludes service Y" primitive), so
true exclusivity comes from both being profile-gated with no shared
default profile, not from one excluding the other directly.

Mirrors Helm's same-Service-name trick so alerting's API_QUERY_URL and
web's VITE_API_BASE_URL need zero conditional logic either way:
enterprise-api now maps host port 8080 (was 8083, its own binary
default -- overridden via HTTP_LISTEN_ADDR) and carries a
`networks.default.aliases: [api]` entry, so whichever binary is
actually running answers on the same compose-network hostname and host
port. alerting's and web's depends_on for api/enterprise-api are now
`required: false` (Compose's supported "optional dependency" shape) --
without it, compose errors on the inactive one rather than just
skipping it, since depends_on doesn't otherwise know about profiles.

Verified for real in this environment via `docker compose config`
(renders and validates the merged YAML without needing a daemon):
confirmed api/enterprise-api never both appear in --services output
for either profile selection, confirmed enterprise-api's rendered
block has port 8080/alias "api"/HTTP_LISTEN_ADDR ":8080" when the
enterprise profile is active, and confirmed `docker compose run
enterprise-api ...`/`docker compose build enterprise-api` (used by
enterprise/README.md's and phase-4-runbook.md's provisioning steps)
still work regardless of the active profile -- explicit service
references bypass profile filtering, confirmed by the commands
reaching a daemon-connection permission error rather than a
profile-resolution error. Not verified: an actual `docker compose up`
against a real daemon, still unavailable in this environment.

Docs updated in lockstep -- CLAUDE.md, threat-model.md (including its
summary table), phase-4-runbook.md (new §10a, §8's provisioning
commands updated for the new port/profile), enterprise/README.md.

.env

@@ -0,0 +1,10 @@
+# Selects which query-serving binary docker-compose.yml runs by
+# default: "single-tenant" (api, Phase 0-3 behavior) or "enterprise"
+# (enterprise-api, Phase 4 multi-tenant) -- mutually exclusive, the same
+# choice Helm's enterprise.enabled flag makes for a real cluster
+# (deploy/helm/sentry/templates/api.yaml vs enterprise-api.yaml). See
+# the api/enterprise-api service definitions in docker-compose.yml.
+#
+# Override per-invocation without editing this file:
+#   COMPOSE_PROFILES=enterprise docker compose up
+COMPOSE_PROFILES=single-tenant