docs: add configuration documentation WIP

docs/SERVICE_CONFIG.md

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+# Service Integration & Authentication Configuration
+
+This document covers service-specific configuration, authentication, and metadata integration options.
+
+## services (dict)
+
+Configuration data for each Service. The Service will have the data within this section merged into the `config.yaml`
+before provided to the Service class.
+
+Think of this config to be used for more sensitive configuration data, like user or device-specific API keys, IDs,
+device attributes, and so on. A `config.yaml` file is typically shared and not meant to be modified, so use this for
+any sensitive configuration data.
+
+The Key is the Service Tag, but can take any arbitrary form for its value. It's expected to begin as either a list or
+a dictionary.
+
+For example,
+
+```yaml
+NOW:
+  client:
+    auth_scheme: MESSO
+    # ... more sensitive data
+```
+
+---
+
+## credentials (dict[str, str|list|dict])
+
+Specify login credentials to use for each Service, and optionally per-profile.
+
+For example,
+
+```yaml
+ALL4: jane@gmail.com:LoremIpsum100 # directly
+AMZN: # or per-profile, optionally with a default
+  default: jane@example.tld:LoremIpsum99 # <-- used by default if -p/--profile is not used
+  james: james@gmail.com:TheFriend97
+  john: john@example.tld:LoremIpsum98
+NF: # the `default` key is not necessary, but no credential will be used by default
+  john: john@gmail.com:TheGuyWhoPaysForTheNetflix69420
+```
+
+The value should be in string form, i.e. `john@gmail.com:password123` or `john:password123`.
+Any arbitrary values can be used on the left (username/password/phone) and right (password/secret).
+You can also specify these in list form, i.e., `["john@gmail.com", ":PasswordWithAColon"]`.
+
+If you specify multiple credentials with keys like the `AMZN` and `NF` example above, then you should
+use a `default` key or no credential will be loaded automatically unless you use `-p/--profile`. You
+do not have to use a `default` key at all.
+
+Please be aware that this information is sensitive and to keep it safe. Do not share your config.
+
+---
+
+## tmdb_api_key (str)
+
+API key for The Movie Database (TMDB). This is used for tagging downloaded files with TMDB,
+IMDB and TVDB identifiers. Leave empty to disable automatic lookups.
+
+To obtain a TMDB API key:
+
+1. Create an account at <https://www.themoviedb.org/>
+2. Go to <https://www.themoviedb.org/settings/api> to register for API access
+3. Fill out the API application form with your project details
+4. Once approved, you'll receive your API key
+
+For example,
+
+```yaml
+tmdb_api_key: cf66bf18956kca5311ada3bebb84eb9a # Not a real key
+```
+
+**Note**: Keep your API key secure and do not share it publicly. This key is used by the core/utils/tags.py module to fetch metadata from TMDB for proper file tagging.
+
+---
+
+## simkl_client_id (str)
+
+Client ID for SIMKL API integration. SIMKL is used as a metadata source for improved title matching and tagging,
+especially when a TMDB API key is not configured.
+
+To obtain a SIMKL Client ID:
+
+1. Create an account at <https://simkl.com/>
+2. Go to <https://simkl.com/settings/developer/>
+3. Register a new application to receive your Client ID
+
+For example,
+
+```yaml
+simkl_client_id: "your_client_id_here"
+```
+
+**Note**: While optional, having a SIMKL Client ID improves metadata lookup reliability. SIMKL serves as an alternative or fallback metadata source to TMDB. This is used by the `core/utils/tags.py` module.
+
+---
+
+## title_cache_enabled (bool)
+
+Enable/disable caching of title metadata to reduce redundant API calls. Default: `true`.
+
+---
+
+## title_cache_time (int)
+
+Cache duration in seconds for title metadata. Default: `1800` (30 minutes).
+
+---
+
+## title_cache_max_retention (int)
+
+Maximum retention time in seconds for serving slightly stale cached title metadata when API calls fail.
+Default: `86400` (24 hours). Effective retention is `min(title_cache_time + grace, title_cache_max_retention)`.
+
+---