refactor: consolidate oidc_config and admin_thresholds into generic server_config table

Single JSONB key-value table replaces two singleton config tables, making
future config types trivial to add. Also fixes pre-existing IT failures:
Flyway URL not overridden by Testcontainers, threshold test ordering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/resources/db/migration/V4__server_config.sql

@@ -0,0 +1,36 @@
+-- =============================================================
+-- Consolidate oidc_config + admin_thresholds → server_config
+-- =============================================================
+
+CREATE TABLE server_config (
+    config_key  TEXT        PRIMARY KEY,
+    config_val  JSONB       NOT NULL,
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_by  TEXT
+);
+
+-- Migrate existing oidc_config row (if any)
+INSERT INTO server_config (config_key, config_val, updated_at)
+SELECT 'oidc',
+       jsonb_build_object(
+           'enabled',          enabled,
+           'issuerUri',        issuer_uri,
+           'clientId',         client_id,
+           'clientSecret',     client_secret,
+           'rolesClaim',       roles_claim,
+           'defaultRoles',     to_jsonb(default_roles),
+           'autoSignup',       auto_signup,
+           'displayNameClaim', display_name_claim
+       ),
+       updated_at
+FROM oidc_config
+WHERE config_id = 'default';
+
+-- Migrate existing admin_thresholds row (if any)
+INSERT INTO server_config (config_key, config_val, updated_at, updated_by)
+SELECT 'thresholds', config, updated_at, updated_by
+FROM admin_thresholds
+WHERE id = 1;
+
+DROP TABLE oidc_config;
+DROP TABLE admin_thresholds;