Add examples/demo: one of every CRD, plus a script to fire alerts at it

A self-contained demo kit: a TerdutServer against a throwaway, bare
Postgres (bring-your-own DSN -- simplest path to stand up from nothing,
ROADMAP.md Stage 1's own note), two TerdutTeams, and each team's own
TerdutEscalationRule/TerdutDeadmanSwitch/TerdutAlertSource, so every CRD
this operator manages is exercised together rather than in isolation the
way config/samples' one-of-each already does.

fire-alerts.sh sends terdut-server's own amPayload/amAlert shape (read
from internal/api/alertmanager.go in that repo, not guessed from its
docs) at whichever TerdutAlertSource's generated webhook Secret it reads
the key out of -- high-cpu/disk-full/pod-crash scenarios to open and
resolve incidents, and a heartbeat scenario matching each team's dead
man's switch matcher, so stopping it demonstrates the switch noticing
silence on its own.

Verified server-side (kubectl apply --dry-run=server -k examples/demo)
against this operator's own dev cluster, which already has these CRDs
installed: every object validates. The one warning that cluster's
"restricted" PodSecurity raises (postgres:17-alpine's entrypoint needs to
start as root before it drops privileges itself) is noted inline in
00-postgres.yaml rather than worked around -- not a real production
pattern, and this Postgres exists only to be thrown away with the rest of
the demo namespace.

README.md walks through: applying, watching status, why a few early
CrashLoopBackOff restarts on terdut-demo itself are expected (this
operator's Deployment template has no wait-for-postgres init container
yet, unlike charts/terdut-server's chart as of v0.33.2), reaching the web
UI (port-forward -- spec.networking.hostname is accepted but nothing
creates an HTTPRoute for it yet), turning on open signup with the
operator's own generated admin token since the bootstrap-created account
has no password, firing alerts, and tearing down.

README.md

@@ -34,3 +34,11 @@ DESIGN.md §4.1, §4.6.
   - `teamRef` (DESIGN.md §4.5)
   - URL/key are generated by the server at creation and surfaced only via a
     generated Secret, never set explicitly
+
+## Demo
+
+[`examples/demo`](./examples/demo) wires one of every CRD above together
+— two teams, each with an escalation rule, a dead man's switch and an
+alert source — plus a script that fires synthetic Alertmanager webhooks
+at it, so you can watch real incidents open, escalate and resolve without
+a real Alertmanager anywhere in the picture.