fix: warn on malformed .env lines instead of silently skipping them

[spartan124]

Closes #172

📌 Description

Previously, internal/config/dotenv.go utilized godotenv.Load() and silently ignored any
parsing errors. If a .env file contained a malformed line (e.g., missing =,
unterminated quote), a typo would silently cause variables to remain unset rather than
failing fast, surfacing later as confusing downstream errors.

This PR implements a robust warn-and-skip parser strategy. Instead of entirely aborting
.env loading, malformed lines are now isolated, loudly logged to stderr with a
descriptive warning, and skipped, allowing the remainder of the valid .env
configuration to be safely loaded.

🧩 Changes made

• Warn-and-Skip Parser Strategy: Replaced the bulk godotenv.Load() call with a custom
bufio.Scanner line-by-line wrapper.
• Enhanced Warning Logs: Malformed lines (missing = or unterminated quotes) now
correctly trigger a detailed WARNING log indicating the exact line context and file
path.
• Robust Duplicate Key Handling: Enforces explicit last-wins behavior within the same .
env file prior to updating the OS environment (which still safely ignores already-
exported OS variables).
• Added Full Test Suite: Created internal/config/dotenv_test.go covering edge cases to
guarantee stability.

🧪 Testing and Acceptance

go test ./internal/config/... -v -cover shows comprehensive test coverage over 95%.

The test suite explicitly verifies:

• ✅ Valid .env files parse identically to before.
• ✅ Missing = signs log a warning and correctly skip the single broken line.
• ✅ Unterminated quotes log a warning and properly capture block-level failures
without panicking.
• ✅ Duplicate keys resolve explicitly to a last-wins outcome within the file.
• ✅ Empty files and comment-only lines load gracefully.
• ✅ Existing OS environment variables are not overwritten.

Test Output

=== RUN   TestLoadDotenv                                                             
=== RUN   TestLoadDotenv/Valid_file
=== RUN   TestLoadDotenv/Missing_equals_sign
=== RUN   TestLoadDotenv/Unterminated_quote 
=== RUN   TestLoadDotenv/Duplicate_key_(last-wins)
=== RUN   TestLoadDotenv/Empty_file
=== RUN   TestLoadDotenv/Comment-only_line
=== RUN   TestLoadDotenv/Mixed_with_comments
=== RUN   TestLoadDotenv/Does_not_overwrite_existing_OS_env
--- PASS: TestLoadDotenv (0.21s)
PASS
ok      github.com/jagadeesh/grainlify/backend/internal/config    3.252s

🔒 Security Notes

Verified that no sensitive defaults are implicitly used as fallbacks if parsing fails.
This change only adds verbosity to local dev tooling; downstream validation layers
continue to gate execution on missing required variables.