datum-gateway: convert to a role, de-Uptime-Kuma the health check

802-line playbook becomes 68 lines (three plays: the role, the Caddy dashboard,
the Stratum socket proxy) plus a 345-line role. datum_gateway_vars.yml is
deleted; its content is the role's defaults.

Verified after a real run with zero miners connected: datum-gateway restarted
cleanly onto the reformatted config, deployed config.json semantically identical
to what was there (pool_address bc1qvrj3g84..., pool_pass_* false, ports
unchanged), health check timer firing, and the Knots side untouched - bitcoind
still up since 2026-08-19 with blocknotify intact.

TWO PIECES OF DRIFT WHERE THE NODE WAS RIGHT, both confirmed with the operator:

- datum_mining_address: the vault held bc1qdse9dsg... while the node had been
  mining to bc1qvrj3g... since 2026-08-08. This is WHERE BLOCK REWARDS ARE PAID.
  And unlike fulcrum and bitcoin-knots, the `Restart datum-gateway` handler here
  was never gated, so the stale value would have applied immediately rather than
  sitting inert on disk.
- pool_pass_workers / pool_pass_full_users: false on the node, true in the vars
  file.

Both corrected in the vault and role defaults with notes recording why.

Comparing this config needs semantics, not text: the live file is single-line
JSON and the template renders pretty-printed, so a textual diff is pure noise.
Rendering it and comparing parsed JSON is what surfaced both differences.

config.json carries bitcoind.rpcpassword and api.admin_password, and --diff
prints rendered content - so `--check --diff` put them on the terminal. The task
now sets diff: false by default (-e datum_reveal_config=true to opt in). Those
two should be rotated.

I also mis-reported pool_pass_workers/pool_pass_full_users as exposed credentials
because my masking matched "pass" in the key name. They are BOOLEANS, and
mining.pool_address is a Bitcoin address, public by nature. Only the two real
passwords above were exposed.

`Configure cmake build` and `Compile datum_gateway` are bare command: tasks with
no changed_when, so they recompile on every run. The build is reproducible -
Install datum_gateway binary sees identical content and leaves the installed
binary's timestamp alone - but it is wasted work each time. Documented as the
idempotent floor.

Ownership parity checked mechanically against `git show HEAD:` keyed by task
name: 7/7 match, 9 Kuma tasks dropped.

This completes Plan 6 Stage 2: all six services in the list are roles.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/roles/datum_gateway/templates/config.json.j2

@@ -0,0 +1,35 @@
+{
+    "bitcoind": {
+        "rpcuser": "{{ bitcoin_rpc_user }}",
+        "rpcpassword": "{{ bitcoin_rpc_password }}",
+        "rpcurl": "{{ datum_bitcoin_rpc_url }}",
+        "notify_fallback": true
+    },
+    "stratum": {
+        "listen_port": {{ datum_gateway_stratum_port }},
+        "vardiff_min": {{ datum_vardiff_min }}
+    },
+    "mining": {
+        "pool_address": "{{ datum_mining_address }}",
+        "coinbase_tag_primary": "{{ datum_coinbase_tag_primary }}",
+        "coinbase_tag_secondary": "{{ datum_coinbase_tag_secondary }}"
+    },
+    "api": {
+        "admin_password": "{{ datum_gateway_admin_password }}",
+        "listen_port": {{ datum_gateway_api_port }},
+        "modify_conf": false
+    },
+    "logger": {
+        "log_to_console": true,
+        "log_to_file": true,
+        "log_file": "{{ datum_gateway_log_dir }}/datum_gateway.log",
+        "log_rotate_daily": true,
+        "log_level_console": 2,
+        "log_level_file": 1
+    },
+    "datum": {
+        "pool_pass_workers": {{ datum_pool_pass_workers | lower }},
+        "pool_pass_full_users": {{ datum_pool_pass_full_users | lower }},
+        "pooled_mining_only": {{ datum_pooled_mining_only | lower }}
+    }
+}