fix(mempool): resolve CORE_RPC_HOST to the actual bitcoin node (Knots/Core) (B12)

CORE_RPC_HOST was hardcoded to bitcoin-knots in three env-render paths, so on a
bitcoin-core node (container named bitcoin-core) mempool-api could not reach
Bitcoin RPC. Both node variants are reachable on archy-net by container name —
only the name differs.

- Legacy direct-podman (stacks.rs) and config.rs::get_app_config now use a new
  dependencies::detect_bitcoin_rpc_host() (pure, unit-tested pick_bitcoin_host).
- Quadlet/manifest path (the modern fleet default): add a {{BITCOIN_HOST}}
  derived-env placeholder — HostFacts.bitcoin_host + resolve_derived_env render
  it; prod_orchestrator detects Knots/Core via podman ps, resolved on demand
  only for manifests that use the placeholder. mempool-api manifest moves
  CORE_RPC_HOST from static env to derived_env: {{BITCOIN_HOST}}.

Tests: pick_bitcoin_host (5 cases incl. substring safety), container-crate
resolve_derived_env, and orchestrator mempool_core_rpc_host_follows_bitcoin_node
(core->bitcoin-core, knots->bitcoin-knots). No-regression confirmed: picker
returns bitcoin-knots live on .198. Live bitcoin-core validation pending (no
core node available). Sibling hardcodes (lnd/btcpay/electrumx/fedimint) tracked
as B12b.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/mempool-api/manifest.yml

@@ -8,6 +8,12 @@ app:
     image: git.tx1138.com/lfg2025/mempool-backend:v3.0.0
     pull_policy: if-not-present
     network: archy-net
+    # CORE_RPC_HOST must follow the node's actual Bitcoin container — Knots or
+    # Core — resolved at apply time from host facts (B12). Hardcoding either
+    # breaks mempool's RPC connection on the other.
+    derived_env:
+      - key: CORE_RPC_HOST
+        template: "{{BITCOIN_HOST}}"
     secret_env:
       - key: CORE_RPC_PASSWORD
         secret_file: bitcoin-rpc-password
@@ -47,7 +53,6 @@ app:
     - ELECTRUM_HOST=electrumx
     - ELECTRUM_PORT=50001
     - ELECTRUM_TLS_ENABLED=false
-    - CORE_RPC_HOST=bitcoin-knots
     - CORE_RPC_PORT=8332
     - CORE_RPC_USERNAME=archipelago
     - DATABASE_ENABLED=true