Add Grafana Alloy and Loki for unified observability (#11)

## Summary
- Add Grafana Alloy to replace node_exporter for metrics collection
- Add Loki for log aggregation and storage
- Configure Alloy to collect logs from all services (grafana, forgejo, prometheus, tailscale, transmission, devpi, kiwix, borgmatic)
- Update Prometheus to accept metrics via remote_write
- Add Loki datasource to Grafana

## Test plan
- [ ] Run \`mise run provision-indri -- --check --diff\` to verify changes
- [ ] Apply with \`mise run provision-indri\`
- [ ] Verify services: \`mise run indri-services-check\`
- [ ] Check Grafana Explore with Loki datasource
- [ ] Query logs: \`{service="grafana"}\`
- [ ] Verify metrics still flowing to Prometheus dashboards

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.tail8d86e.ts.net/eblume/blumeops/pulls/11

ansible/roles/loki/defaults/main.yml

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+---
+# Loki configuration
+
+# Server settings
+loki_http_port: 3100
+
+# Storage paths
+loki_data_dir: /opt/homebrew/var/loki
+loki_config_file: /opt/homebrew/etc/loki-local-config.yaml
+
+# Retention settings
+loki_retention_period: 744h  # 31 days

ansible/roles/loki/handlers/main.yml

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+---
+- name: restart loki
+  ansible.builtin.command: brew services restart loki

ansible/roles/loki/meta/main.yml

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+---
+dependencies: []

ansible/roles/loki/tasks/main.yml

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+---
+# Loki installation and configuration
+
+- name: Install loki via homebrew
+  community.general.homebrew:
+    name: loki
+    state: present
+
+- name: Ensure loki data directory exists
+  ansible.builtin.file:
+    path: "{{ loki_data_dir }}"
+    state: directory
+    mode: '0755'
+
+- name: Ensure loki chunks directory exists
+  ansible.builtin.file:
+    path: "{{ loki_data_dir }}/chunks"
+    state: directory
+    mode: '0755'
+
+- name: Ensure loki rules directory exists
+  ansible.builtin.file:
+    path: "{{ loki_data_dir }}/rules"
+    state: directory
+    mode: '0755'
+
+- name: Deploy loki configuration
+  ansible.builtin.template:
+    src: loki-config.yaml.j2
+    dest: "{{ loki_config_file }}"
+    mode: '0644'
+  notify: restart loki
+
+- name: Ensure loki service is started
+  ansible.builtin.command: brew services start loki
+  register: brew_start
+  changed_when: "'Successfully started' in brew_start.stdout"
+  failed_when: false

ansible/roles/loki/templates/loki-config.yaml.j2

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+# {{ ansible_managed }}
+# Loki configuration for single-node deployment
+
+auth_enabled: false
+
+server:
+  http_listen_port: {{ loki_http_port }}
+  grpc_listen_port: 9096
+
+common:
+  instance_addr: 127.0.0.1
+  path_prefix: {{ loki_data_dir }}
+  storage:
+    filesystem:
+      chunks_directory: {{ loki_data_dir }}/chunks
+      rules_directory: {{ loki_data_dir }}/rules
+  replication_factor: 1
+  ring:
+    kvstore:
+      store: inmemory
+
+query_range:
+  results_cache:
+    cache:
+      embedded_cache:
+        enabled: true
+        max_size_mb: 100
+
+schema_config:
+  configs:
+    - from: 2024-01-01
+      store: tsdb
+      object_store: filesystem
+      schema: v13
+      index:
+        prefix: index_
+        period: 24h
+
+storage_config:
+  tsdb_shipper:
+    active_index_directory: {{ loki_data_dir }}/tsdb-index
+    cache_location: {{ loki_data_dir }}/tsdb-cache
+
+limits_config:
+  retention_period: {{ loki_retention_period }}
+
+compactor:
+  working_directory: {{ loki_data_dir }}/compactor
+  compaction_interval: 10m
+  retention_enabled: true
+  retention_delete_delay: 2h
+  retention_delete_worker_count: 150
+  delete_request_store: filesystem