docs: Add comprehensive documentation to .env.example

[ShashwatpSingh]

Summary

This PR adds comprehensive documentation to .env.example, making environment setup self-documenting and dramatically improving the developer experience for new contributors.

Problem

The existing .env.example had 40+ environment variables with zero documentation. New contributors had to:

• Guess what each variable does
• Search through code to understand requirements
• Trial-and-error to find working values

Solution

Added detailed documentation including:

Structure

• Section headers with visual separators (Database, Security, Cache, etc.)
• Logical grouping of related variables
• Clear [REQUIRED] / [OPTIONAL] marking

Per-Variable Documentation

• What each variable does
• Expected format and examples
• Default values and valid options
• Security warnings where appropriate
• Links to external setup docs (Upstash, OAuth, etc.)

Security Improvements

• Warnings about changing default secrets in production
• Notes about minimum lengths for keys
• Explanations of security parameters (scrypt, pepper)

Preview

# BEFORE (confusing)
OTM_SCRYPT_N=16384
OTM_SCRYPT_R=8

# AFTER (clear)
# Scrypt parameters for password hashing (adjust based on your server's CPU)
# These are secure defaults - only change if you know what you're doing
OTM_SCRYPT_N=16384      # CPU/memory cost parameter (higher = more secure, slower)
OTM_SCRYPT_R=8          # Block size parameter
OTM_SCRYPT_P=1          # Parallelization parameter
OTM_SCRYPT_MAXMEM=67108864  # Max memory in bytes (64MB)

Impact

• Faster onboarding: New contributors can self-serve setup
• Fewer issues: Less "how do I configure X?" questions
• Better security: Clear warnings about production secrets
• Self-hosting ready: Complete reference for production deployment

Testing

• Verified all original variables preserved
• Checked formatting renders correctly
• Validated comments don't break any parsers
• Confirmed no sensitive values exposed

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Related: This directly addresses the setup friction mentioned in [Discord/onboarding feedback].

No breaking changes - purely additive documentation.

[vercel]

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