Add service infrastructure for long-running deployment

- Add configuration system (config/*.exs, OrgGarden.Config)
- Refactor supervision tree with DynamicSupervisor and Registry
- Add OrgGarden.Server for serve mode lifecycle management
- Add health check HTTP endpoints (Bandit/Plug on :9090)
- Add telemetry events for export and watcher operations
- Implement graceful shutdown with SIGTERM handling
- Add Mix Release support with overlay scripts
- Add NixOS module for systemd service deployment
- Update documentation with service usage

config/config.exs

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+import Config
+
+# Default configuration values
+# These are overridden by config/runtime.exs in releases
+
+config :org_garden,
+  zotero_url: "http://localhost:23119",
+  citation_mode: :warn,
+  http_port: 8080,
+  ws_port: 3001,
+  health_port: 9090
+
+config :logger, :console,
+  format: "$time $metadata[$level] $message\n",
+  metadata: [:module]
+
+# Import environment specific config
+import_config "#{config_env()}.exs"

config/dev.exs

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+import Config
+
+# Development-specific configuration
+config :logger, level: :debug

config/prod.exs

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+import Config
+
+# Production-specific configuration
+# Most config comes from runtime.exs via environment variables
+
+config :logger, level: :info

config/runtime.exs

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+import Config
+
+# Runtime configuration from environment variables
+# This file is executed at runtime (not compile time)
+
+defmodule RuntimeConfig do
+  def parse_int(nil, default), do: default
+  def parse_int(val, _default) when is_integer(val), do: val
+
+  def parse_int(val, default) when is_binary(val) do
+    case Integer.parse(val) do
+      {int, ""} -> int
+      _ -> default
+    end
+  end
+
+  def parse_citation_mode("silent"), do: :silent
+  def parse_citation_mode("strict"), do: :strict
+  def parse_citation_mode(_), do: :warn
+end
+
+# Core paths
+if quartz_path = System.get_env("QUARTZ_PATH") do
+  config :org_garden, quartz_path: quartz_path
+end
+
+config :org_garden,
+  node_path: System.get_env("NODE_PATH", "node"),
+  notes_dir: System.get_env("NOTES_DIR"),
+  output_dir: System.get_env("OUTPUT_DIR")
+
+# Citation configuration
+config :org_garden,
+  zotero_url: System.get_env("ZOTERO_URL", "http://localhost:23119"),
+  bibtex_file: System.get_env("BIBTEX_FILE"),
+  citation_mode: RuntimeConfig.parse_citation_mode(System.get_env("CITATION_MODE"))
+
+# Server ports
+config :org_garden,
+  http_port: RuntimeConfig.parse_int(System.get_env("PORT"), 8080),
+  ws_port: RuntimeConfig.parse_int(System.get_env("WS_PORT"), 3001),
+  health_port: RuntimeConfig.parse_int(System.get_env("HEALTH_PORT"), 9090)

config/test.exs

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+import Config
+
+# Test-specific configuration
+config :logger, level: :warning